Declared · Counted · Finite

This class contains time units that exist only by human agreement, are defined by counted duration, and have clear operational boundaries. These units are created to coordinate activity across people, systems, and records.

Although many calendar units are historically derived from astronomical cycles, their current form and use are conventional, not natural. Their lengths are fixed by rule, not by physical necessity.

Clock / Time-of-Day Units

clock minute
clock hour
quarter-hour
half-hour
three-quarter hour
minute-of-hour
hour-of-day
a.m. half-day
p.m. half-day
UTC minute
UTC hour
UTC day
local civil hour
local civil day
standard-time hour
daylight-saving-time hour
repeated clock hour
skipped clock hour

Day / Date Units

calendar day
civil day
UTC date
local date
day-of-month
day-of-year
ordinal day
weekday
weekend day, where defined
legal holiday
public holiday
bank holiday
business day
court day
school day, when rule-defined
trading day
settlement day

Week Units

seven-day week
calendar week
ISO week
Sunday–Saturday week
Monday–Sunday week
week-of-year
ISO week-year
biweek
fortnight
pay week
workweek, if defined as a date block
retail week
broadcast week

Month Units

calendar month
Gregorian month
January
February
February in a leap year
30-day month
31-day month
fiscal month
retail month
lunar-calendar month, as calendar unit
Hebrew calendar month
Islamic civil/tabular month
Chinese lunisolar month
leap month
intercalary month
embolismic month
Adar II

Year Units

calendar year
Gregorian year
common year
leap year
Julian calendar year
Revised Julian year
ISO week-numbering year
fiscal year
tax year
reporting year
program year
benefit year
policy year
membership year, one bounded occurrence
lease year, one bounded occurrence
anniversary year

Larger Year-Block Units

biennium
triennium
quadrennium
quinquennium
lustrum
sexennium
septennium
octennium
decade
calendar decade
century
calendar century
millennium
calendar millennium
Julian century
Julian millennium

Intercalation Units

leap day
February 29
intercalary day
epagomenal day
leap week
53rd ISO week
53-week fiscal year
leap month
embolismic year
common year
leap year

Time-Zone / Civil-Time Units

time-zone day
standard-time day
daylight-saving day
DST transition day
spring-forward day
fall-back day
repeated local hour
skipped local hour
UTC offset interval
zone-local calendar day
International Date Line date boundary

Administrative / Legal / Financial Units

tax year
fiscal year
fiscal quarter
calendar quarter
reporting quarter
fiscal month
pay period
weekly pay period
biweekly pay period
semimonthly pay period
billing period
billing cycle, one occurrence
statement period
premium period
grace period
notice period
appeal period
comment period
statutory waiting period
limitation period
fixed election cycle
fixed constitutional term length

Technical / Timestamp / Recordkeeping Units

ISO 8601 date
RFC 3339 timestamp date
ordinal date
week date
Julian Day Number
Julian day, as count unit
Modified Julian Day
Rata Die day
Unix/POSIX day
Unix epoch day
GPS week
NTP era
database date partition
log-rotation day
cron day field
cron week field
cron month field

Best Examples to Use

Example	Why it is strong
clock minute	Declared 60-second civil clock unit.
clock hour	Declared 60-minute civil clock unit.
civil day	Midnight-to-midnight rule, not natural Sun return.
UTC day	Global rule-bounded date unit; leap seconds prove its conventional structure.
calendar week	Seven-day declared cycle with no natural astronomical closure.
ISO week	Standardized finite week unit.
calendar month	Rule-defined month, not lunar phase cycle.
Gregorian year	Rule-defined civil year.
leap year	Artificial correction year by calendar rule.
leap day	Inserted civil-calendar correction unit.
fiscal year	Finite accounting year, declared by institution/law.
tax year	Finite legal reporting year.
business day	Counted civil/legal day type.
Julian Day Number	Declared continuous day-count system.
Unix/POSIX day	Technical rule-defined software day.
DST transition day	Rule-created anomaly in local civil time.

Prompt:

A clean vector brand logo for the time class “Declared · Counted · Finite,” representing calendar and civil time. Use a circular time-ring built from civic gold, emerald green, and ember red. The icon should combine a human-made seal or civic marker, a precise grid of counted calendar ticks, and a firmly closed square or bracketed boundary inside the circular ring. Minimal flat vector logo, centered icon, consistent stroke weight, ivory background, no text, no letters, no numbers, premium institutional identity-system design.

Boundary of This Class

Included:

Units defined by human agreement
Units with fixed counted duration
Units used for coordination and record-keeping

Excluded:

Units defined by natural cycles (solar day, tropical year)
Units defined by institutional authority beyond the calendar (fiscal years)
Units defined by role rather than duration (deadlines)

Declared · Counted · Finite

Calendar / Civil Time Units

This class contains rule-made coordination units. They exist because a society, state, standard, institution, calendar system, or technical protocol says they exist.

They are declared because their boundaries are conventional.
They are counted because they are made from counted smaller units: seconds, minutes, hours, days, weeks, months, or years.
They are finite because each occurrence has a definite operational start and end.

The clean name for this class is:

Civil-Calendar Count Units

or, more formally:

Declared Finite Coordination Units

The important correction:

A Calendar or Civil Time unit does not need to have the same number of seconds every time. It needs to have a rule that determines its counted length and boundary.

So a calendar month belongs here even though February, March, and April have different lengths.
A local civil day belongs here even though daylight-saving changes can make a local day 23 or 25 hours.
A UTC day belongs here even though leap seconds can make some UTC days 86,401 seconds.
The unit is not natural. The rule closes it.

NIST notes that minutes and hours are not official SI units, but are accepted for use; it also states the ordinary base-60 rule that 60 seconds make a minute and 60 minutes make an hour. That is exactly Calendar or Civil Time Units territory: culturally and conventionally accepted coordination units built on counted duration.

Membership Test for Calendar or Civil Time Units

A candidate belongs here only if it passes this test:

Test	Requirement
Declared	The unit exists by calendar, legal, civil, administrative, or technical rule.
Counted	Its length is determined by counting smaller declared or measured units.
Finite	Each occurrence has an operationally closed start and end.
Civil / calendar function	It coordinates records, schedules, dates, timestamps, contracts, law, administration, or shared social time.

The shortest rule:

If the unit is a rule-bounded slot in a calendar, clock, timestamp, legal schedule, accounting system, or civil time standard, it belongs in Calendar or Civil Time Units.

The Core Difference from Physical or Fundamental Time Units, Astronomical Time Units, Social or Institutional Time Units, and Logical or Structural Time Units

Class	What defines the unit	Example
Physical / Fundamental	Fixed measured duration	second, millisecond, nanosecond
Astronomical	Natural celestial return	apparent solar day, sidereal year, synodic month
Calendar / Civil	Declared counted rule	clock minute, civil day, calendar week, Gregorian year
Social / Institutional	Open-ended counted status or continuity	tenure, membership duration, subscription tenure
Logical / Structural	Rule-defined role or process step	turn, round, phase, iteration

The class is not “whatever humans talk about.” It is specifically finite counted coordination time.

Important contrasts:

Do not confuse	Correct distinction
60 seconds vs minute	60 seconds is counted physical duration; minute is a conventional clock unit.
86,400 seconds vs civil day	86,400 seconds is a duration; civil day is a rule-bounded date unit.
apparent solar day vs civil day	Apparent solar day is natural Sun return; civil day is midnight-to-midnight by rule.
synodic month vs calendar month	Synodic month is lunar phase return; calendar month is a rule-defined date block.
tropical year vs Gregorian year	Tropical year is seasonal astronomical return; Gregorian year is a calendar rule.
membership tenure vs membership year	Tenure may continue indefinitely; one membership year is a finite counted interval.
semester as academic role vs half-year	Semester usually functions structurally; half-year is counted calendar duration.

A. Clock-Level Civil Units

These are the basic civil clock units. They are not fundamental physical units, even though they are expressed through seconds.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
clock minute	60 seconds, normally	Declared clock unit; counted and finite.
clock hour	60 minutes	Declared clock unit; counted and finite.
quarter-hour	15 minutes	Conventional scheduling unit.
half-hour	30 minutes	Conventional scheduling unit.
three-quarter hour	45 minutes	Conventional clock subdivision.
civil time-of-day second	timestamp field within a minute	The second itself is Physical or Fundamental Time Units, but the “second field” in a clock minute is Calendar or Civil Time Units.
minute-of-hour	00–59	Timestamp/calendar field, rule-bounded.
hour-of-day	00–23, or sometimes 1–12 in 12-hour systems	Clock field, not natural time.
a.m. / p.m. half-day	12-hour clock division	Conventional civil clock partition.
noon-to-midnight half-day	12 hours by clock rule	Declared clock block.
UTC minute	normally 60 seconds; leap-second minute can have 61 seconds	Rule-governed finite minute.
UTC hour	60 UTC minutes	Rule-governed finite clock hour.
UTC day	normally 86,400 seconds; leap-second days may differ	Rule-bounded UTC date day.
local civil hour	hour in a local time zone	Declared by local civil-time rule.
local civil day	local midnight to next local midnight	Rule-bounded date unit; may vary under DST or date-line changes.

Leap seconds prove the point. NIST says UTC is based on TAI but adjusted by leap seconds, and gives the positive leap-second sequence as 23h 59m 59s – 23h 59m 60s – 00h 00m 00s. That means the civil timestamp rule, not natural duration alone, determines the unit boundary.

Best examples from this group:

clock minute, clock hour, quarter-hour, UTC day, local civil day.

B. Calendar Day and Date Units

These are the central Calendar or Civil Time Units examples. They are not natural days. They are date-bearing civil units.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
calendar day	one date unit in a calendar	Declared date slot.
civil day	midnight-to-midnight in a jurisdiction/time zone	Clear civil boundary.
UTC date	00:00:00 UTC to next UTC date boundary	Global technical/civil date unit.
local date	date under local time-zone law/rule	Declared by local civil time.
day-of-month	numbered date within a month	Calendar-count field.
day-of-year / ordinal day	001–365 or 001–366	Calendar-count field.
weekday	Monday, Tuesday, etc.	Declared weekly calendar position.
weekend day, where legally/culturally defined	Saturday/Sunday or local equivalent	Calendar/social classification; finite day unit.
legal holiday	declared civil date	Rule-bounded civil day.
public holiday	declared public calendar day	Civil coordination unit.
bank holiday	declared financial/legal day	Civil-financial calendar unit.
business day	counted eligible day under a jurisdiction or contract	Not natural; finite counted civil/legal unit.
court day	eligible day for legal filing/procedure	Declared civil-legal count unit.
school day, when defined by statute/calendar	counted instructional day	Declared finite calendar unit.
trading day	exchange-defined eligible date	Civil/market calendar unit, not astronomical.
settlement day	financial-system counted day	Rule-bounded civil-financial unit.

Important distinction:

Item	Class
apparent solar day	Astronomical Time Units
civil day	Calendar or Civil Time Units
business day	Calendar or Civil Time Units, if rule-defined as a counted calendar day
work shift	usually Logical or Structural Time Units, because it is a structural role in a schedule
employment tenure	Social or Institutional Time Units, because it is open-ended continuity

C. Week Units

The week is one of the purest Calendar or Civil Time Units units because it has no direct astronomical closure. It is a human counting cycle.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
seven-day week	7 calendar days	Declared counted cycle.
calendar week	jurisdiction/calendar-defined week	Finite coordination block.
ISO week	Monday-through-Sunday week-numbering unit	Standardized calendar week.
Sunday–Saturday week	common U.S.-style week convention	Declared weekly boundary.
Monday–Sunday week	ISO/common international convention	Declared weekly boundary.
week-of-year	numbered week field	Calendar-count field.
ISO week-year	52 or 53 ISO weeks	Rule-defined week-based year.
biweek	2 weeks	Declared counted interval.
fortnight	14 days	Conventional counted civil interval.
pay week, if fixed by payroll rule	7-day payroll block	Finite administrative count unit.
workweek, if defined as a date block	e.g., Monday–Friday or Sunday–Saturday	Civil/institutional calendar unit; not biological or astronomical.
retail week	accounting calendar week	Finite business calendar unit.
broadcast week	media scheduling week	Declared coordination unit.

RFC 3339’s ISO 8601 grammar includes date-week values from 01–52 or 01–53, weekday values 1–7, and ordinal day values 001–365/366, showing exactly how civil time becomes a rule-governed counting scheme.

Best examples from this group:

calendar week, ISO week, week-of-year, ISO week-year, fortnight.

D. Month Units

A calendar month is not a lunar month. It is a declared date block.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
calendar month	month under a calendar system	Declared finite date block.
Gregorian month	one of January–December	Rule-defined civil month.
January	31 days in Gregorian calendar	Declared month unit.
February, common year	28 days	Rule-defined civil month.
February, leap year	29 days	Rule-defined civil month.
March	31 days	Declared date block.
April	30 days	Declared date block.
30-day month, where rule-defined	30 calendar days	Conventional month type.
31-day month, where rule-defined	31 calendar days	Conventional month type.
calendar quarter month-position	first/second/third month of a quarter	Calendar accounting unit.
fiscal month	organization/accounting-defined month	Declared finite administrative calendar unit.
retail month	4-4-5, 4-5-4, or similar retail calendar block	Rule-bounded accounting unit.
lunar-calendar month, when used as civil calendar month	rule/authority-defined month	Calendar or Civil Time Units if it is a named calendar unit, not the raw lunar phase cycle.
Hebrew calendar month	rule-governed lunisolar month	Declared calendar unit.
Islamic civil/tabular month	rule-governed lunar calendar month	Declared calendar unit.
Chinese lunisolar calendar month	rule-governed calendar month	Declared calendar unit, even if astronomically informed.
leap month / intercalary month	added by calendar rule	Declared correction unit.
Adar II, in Hebrew leap years	intercalary calendar month	Rule-defined finite month.
embolismic month	inserted month in lunisolar calendars	Declared calendar correction unit.

RFC 3339’s Gregorian profile gives month numbers 01–12 and states that the maximum day-of-month depends on the month and year: February has 28 or 29 days, while other months have 30 or 31. That is Calendar or Civil Time Units exactly: variable length, but rule-determined and finite.

Best examples from this group:

calendar month, Gregorian month, February in a leap year, fiscal month, leap month.

Important distinction:

Item	Class
synodic month / lunation	Astronomical Time Units
calendar month	Calendar or Civil Time Units
Ramadan as a sacred narrative period	Historical or Narrative Time Units
Ramadan as a calendar month in a rule-governed calendar	Calendar or Civil Time Units
month-to-month membership tenure	Social or Institutional Time Units if open-ended continuity is the point

E. Year Units

A civil year is not the same thing as an astronomical year. It may be designed to approximate one, but its actual boundary is declared.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
calendar year	year under a calendar system	Declared finite date block.
Gregorian year	Jan 1–Dec 31 under Gregorian rules	Civil calendar unit.
common year	365 days	Rule-defined non-leap year.
leap year	366 days	Rule-defined year with added day.
Julian calendar year	year under Julian calendar	Declared calendar unit.
Revised Julian year	year under Revised Julian rules	Declared calendar unit.
ISO week-numbering year	52 or 53 weeks	Week-based civil calendar year.
fiscal year	legally/institutionally declared accounting year	Finite counted coordination unit.
tax year	law-defined reporting year	Civil/legal calendar unit.
academic year, if defined strictly as a counted statutory year	declared education calendar year	Calendar or Civil Time Units only when treated as a counted calendar interval.
reporting year	declared recordkeeping year	Finite administrative unit.
program year	grant/program-defined year	Declared finite administrative interval.
benefit year	insurance/benefit-rule year	Declared finite administrative unit.
policy year	insurance contract year	Finite counted contract calendar unit.
membership year	one finite membership interval	Calendar or Civil Time Units for the single bounded unit; Social or Institutional Time Units for ongoing membership tenure.
lease year	one finite contract year	Declared counted legal interval.
anniversary year	date-to-same-date interval	Rule-defined civil interval.

USNO gives the Gregorian leap-year rule: years divisible by 4 are leap years except centurial years not divisible by 400; for example, 1700, 1800, 1900, and 2100 are not leap years, but 1600, 2000, and 2400 are.

Best examples from this group:

Gregorian year, common year, leap year, fiscal year, tax year, ISO week-year.

Critical correction:

A fiscal year is not automatically Social or Institutional Time Units just because an institution uses it. If the fiscal year has a fixed start and end, it is Declared · Counted · Finite, so it belongs in Calendar or Civil Time Units. The open-ended thing is the institution’s continuing accounting regime, not one fiscal year.

F. Larger Civil-Calendar Count Units

These are declared groupings of calendar years. They are finite when the counting rule is specified.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
biennium	2 years	Declared counted year-block.
triennium	3 years	Declared counted year-block.
quadrennium	4 years	Declared counted year-block.
quinquennium	5 years	Declared counted year-block.
lustrum	5 years	Conventional counted period.
sexennium	6 years	Declared counted year-block.
septennium	7 years	Declared counted year-block.
octennium	8 years	Declared counted year-block.
decade	10 years	Civil counted block.
calendar decade, e.g. 1990–1999	10-year date block	Finite civil/numeric period.
century	100 years	Rule-defined calendar block.
calendar century	e.g., 1901–2000 or 1900–1999, depending convention	Declared finite block; must specify rule.
millennium	1000 years	Rule-defined calendar block.
calendar millennium	1000-year civil block	Declared finite block.
Julian century	36,525 days	Technical calendar/astronomical convention.
Julian millennium	365,250 days	Technical counted convention.

Important distinction:

Item	Class
calendar century	Calendar or Civil Time Units
“the twentieth century” as cultural-historical narrative	Historical or Narrative Time Units
“the long nineteenth century”	Historical or Narrative Time Units
ten years as pure duration	Physical or Fundamental Time Units-style counted expression
decade as a calendar block	Calendar or Civil Time Units

G. Intercalation Units

These are the correction units inserted by calendar rule. They are extremely strong Calendar or Civil Time Units examples because they are visibly artificial.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
leap day	added calendar day	Declared correction unit.
February 29	Gregorian leap day	Rule-defined finite date.
intercalary day	inserted day by calendar rule	Declared calendar correction.
epagomenal day	extra day outside regular months in some calendars	Declared civil/calendar addition.
leap week	added week in week-based calendars	Rule-defined correction unit.
53rd ISO week	extra week in some ISO week-years	Declared week-calendar correction.
53-week fiscal year	retail/accounting calendar correction	Declared administrative calendar unit.
leap month	added month in lunisolar calendar	Declared calendar correction.
embolismic year	year with an inserted month	Rule-defined lunisolar correction.
common year	year without intercalation	Rule-defined non-leap year.
leap year	year with intercalation	Rule-defined correction year.

Intercalation is where Calendar or Civil Time Units separates most sharply from Astronomical Time Units. The astronomical cycle may motivate the correction, but the unit itself exists because a calendar rule inserts it.

H. Time-Zone and Local-Civil-Time Units

These are declared civil units tied to jurisdictions, offsets, and local clock rules.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
time-zone day	local date under a time-zone rule	Declared local civil unit.
standard-time day	date under standard-time offset	Rule-governed local day.
daylight-saving day	date under DST rule	Rule-governed local day.
DST transition day	day containing clock change	Finite civil anomaly unit.
spring-forward day	day with skipped local time	Declared civil-time effect.
fall-back day	day with repeated local time	Declared civil-time effect.
repeated clock hour	duplicated local hour during fall-back	Rule-defined civil-time interval.
skipped clock hour	omitted local hour during spring-forward	Rule-defined civil-time gap.
UTC offset hour	offset counted from UTC	Declared time-zone coordinate.
zone-local calendar day	local midnight to local midnight	Finite rule-bounded date unit.
International Date Line calendar day boundary	date changes across declared/conventional boundary	Civil date boundary, not natural edge.

IANA states that the time-zone database records local-time history and is updated to reflect political changes to time-zone boundaries, UTC offsets, and daylight-saving rules. That is exactly the “declared” part of this class. USNO also says the International Date Line separates consecutive calendar days as a matter of convenience, has no force in international law, and has changed over time due to governments and local interests.

Best examples from this group:

local civil day, DST transition day, repeated clock hour, skipped clock hour, time-zone day.

I. Legal, Administrative, Financial, and Accounting Count Units

These are not “natural” calendar units, but they still belong in Calendar or Civil Time Units when they are finite, counted, and rule-bounded.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
tax year	statutory reporting year	Declared finite civil/legal unit.
fiscal year	accounting/reporting year	Declared finite administrative unit.
fiscal quarter	one quarter of a fiscal year	Rule-defined accounting unit.
calendar quarter	Jan–Mar, Apr–Jun, etc.	Declared finite calendar block.
reporting quarter	administrative/financial quarter	Rule-bounded reporting unit.
fiscal month	accounting month	Declared finite reporting unit.
pay period	payroll-defined interval	Finite administrative unit.
weekly pay period	7-day payroll block	Declared counted unit.
biweekly pay period	14-day payroll block	Declared counted unit.
semimonthly pay period	two pay periods per month	Declared payroll calendar unit.
billing period	contract-defined charging interval	Finite counted administrative unit.
billing cycle, one occurrence	one bounded charge interval	Calendar or Civil Time Units for one cycle; ongoing subscription tenure is Social or Institutional Time Units.
statement period	financial account reporting interval	Rule-bounded recordkeeping unit.
premium period	insurance billing interval	Contractual counted interval.
grace period, if fixed by days	declared finite legal/contract interval	Counted and closed by rule.
notice period, if fixed by days	e.g., 30 days	Legal counted interval.
appeal period	e.g., 14 or 30 days after judgment	Legal finite count unit.
comment period	administrative-law response window	Declared finite civil interval.
statutory waiting period	rule-defined counted interval	Declared finite civil unit.
limitation period, if fixed	legally counted filing interval	Declared finite legal calendar unit.
election cycle, when legally fixed	e.g., 2-year, 4-year, 6-year cycle	Declared finite civil-political unit.
constitutional term length, when fixed	e.g., 2, 4, or 6 years	Declared counted finite office unit.

Important distinction:

Item	Class
one billing period	Calendar or Civil Time Units
ongoing subscription tenure	Social or Institutional Time Units
one fiscal year	Calendar or Civil Time Units
the institution’s ongoing fiscal regime	Social or Institutional Time Units-ish institutional continuity
one statutory waiting period	Calendar or Civil Time Units
state of emergency with extensions	Social or Institutional Time Units
one fixed office term	Calendar or Civil Time Units
time in office / tenure	Social or Institutional Time Units

J. Technical, Timestamp, and Recordkeeping Units

These are declared counted units used by computing, astronomy, navigation, databases, and protocols. They are not natural cycles.

Unit	Rule-basis	Why it fits Calendar or Civil Time Units
ISO 8601 calendar date	standardized date representation	Declared recordkeeping unit.
RFC 3339 timestamp date	internet timestamp profile	Declared technical date-time unit.
ordinal date	year + day-of-year	Declared calendar count.
week date	week-year + week + weekday	Declared calendar count.
Julian Day Number	count of days from a conventional epoch	Declared finite day-count unit.
Julian day, as count unit	continuous day count	Technical calendar-count unit.
Modified Julian Day	shifted Julian day count	Declared technical day unit.
Rata Die day	day count from conventional epoch	Declared computational calendar unit.
Unix/POSIX day	86,400-second software day convention	Declared technical calendar unit.
Unix epoch day	day counted from 1970-01-01 UTC	Declared computational record unit.
GPS week	week count in GPS time	Declared navigation time unit.
NTP era	fixed technical time-count block	Declared finite protocol unit.
database date partition	one civil-date storage interval	Declared recordkeeping unit.
log-rotation day	one rule-defined logging interval	Technical administrative unit.
cron day/week/month field	scheduled rule field	Declared technical calendar unit.

RFC 3339 explicitly defines an Internet date/time format as a profile of ISO 8601 using the Gregorian calendar, and gives a structured grammar for full dates, time offsets, hours, minutes, seconds, and leap-second behavior.

Best examples from this group:

ISO date, ordinal date, ISO week date, Julian Day Number, Modified Julian Day, Unix/POSIX day, GPS week.

K. Calendar-System Units Beyond the Gregorian Calendar

These belong in Calendar or Civil Time Units when they are used as rule-governed calendar units, even if their original motivation is astronomical, religious, agricultural, or historical.

Calendar system	Calendar or Civil Time Units units
Gregorian calendar	day, week, month, common year, leap year, century, millennium
Julian calendar	Julian day, Julian month, Julian year, Julian leap year
Revised Julian calendar	day, month, year, leap year
ISO week calendar	ISO week, ISO week-year, ISO weekday
Hebrew calendar	calendar day, month, leap month, common year, embolismic year
Islamic civil/tabular calendar	day, month, common year, leap year
Chinese lunisolar calendar	calendar month, leap month, calendar year
Persian / Solar Hijri calendar	day, month, year, leap year
Coptic calendar	month, epagomenal days, year
Ethiopian calendar	month, epagomenal days, year
French Republican calendar	décade, month, complementary days, year
Roman calendar	Kalends/Nones/Ides date positions, intercalary month
Maya Long Count	kʼin, winal, tun, kʼatun, bʼakʼtun as counted calendar units
Mesoamerican 260-day calendar systems	trecena, veintena, named day-cycle positions
Retail 4-4-5 calendar	retail week, retail month, retail quarter, 53-week year
Academic calendar, when reduced to fixed counted blocks	academic year, term dates, instructional days

Caution:

A calendar system can use astronomical observations, but the calendar unit is Calendar or Civil Time Units when the boundary is formalized by a rule, authority, or civil convention. The raw natural cycle remains Astronomical Time Units.

Things That Do Not Belong in Calendar or Civil Time Units

Do not put in Calendar or Civil Time Units	Better class	Why
second, as SI physical unit	Physical or Fundamental Time Units	Defined by physical atomic frequency.
millisecond	Physical or Fundamental Time Units	Exact subdivision of physical second.
86,400 seconds	Physical or Fundamental Time Units-style counted duration expression	A duration, not a civil date unit.
apparent solar day	Astronomical Time Units	Natural Sun-return cycle.
sidereal day	Astronomical Time Units	Natural rotation relative to stars.
synodic month	Astronomical Time Units	Lunar phase-return cycle.
tropical year	Astronomical Time Units	Seasonal astronomical return.
sidereal year	Astronomical Time Units	Orbital return relative to stars.
gestation	Biological Time Units	Biological development unit.
menstrual cycle	Biological Time Units	Biological reproductive rhythm.
sleep cycle	Biological Time Units	Biological sleep-stage sequence.
employment tenure	Social or Institutional Time Units	Open-ended institutional continuity.
membership tenure	Social or Institutional Time Units	Continues until cancellation/removal/death/etc.
state of emergency	Social or Institutional Time Units	Declared but extendable/open-ended.
subscription tenure	Social or Institutional Time Units	Ongoing counted continuity.
turn	Logical or Structural Time Units	Role-based structural unit, not counted duration.
round	Logical or Structural Time Units	Rule-defined action unit, not duration unit.
iteration	Logical or Structural Time Units	Process pass, not calendar duration.
trial	Logical or Structural Time Units	Structural experimental/game unit.
Renaissance	Historical or Narrative Time Units	Historical/narrative period.
Cold War	Historical or Narrative Time Units	Historical interpretation, even if dates are assigned.
Digital Age	Historical or Narrative Time Units	Narrative-historical framing.
Roaring Twenties	Historical or Narrative Time Units	Cultural-historical label, not mere decade.

The Clean Rule for Calendar or Civil Time Units

If the unit is created by rule, built from counted smaller units, and closes at a definite boundary used for coordination or records, it belongs in Calendar or Civil Time Units.

More exact:

Calendar or Civil Time Units contains finite civil, calendar, legal, administrative, financial, and technical time units whose boundaries are not discovered in nature but imposed by a counting system.

The decisive feature is not whether the unit is “important” or “social.” The decisive feature is this:

Can the unit be closed by applying a declared counting rule?

If yes, it is Calendar or Civil Time Units.