Calendar year

CYquarterQ1fourth quarterQ4Q2civil yearfirst quartersecond quarteryear
Generally speaking, a calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days.wikipedia
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Year

myaMay
A year can also be measured by starting on any other named day of the calendar, and ending on the day before this named day in the following year.
A calendar year is an approximation of the number of days of the Earth's orbital period as counted in a given calendar.

Intercalation (timekeeping)

intercalationintercalaryintercalary month
To reconcile the calendar year with the astronomical cycle (which has a fractional number of days) certain years contain extra days ("leap days" or "intercalary days").
The solar or tropical year does not have a whole number of days (it is about 365.24 days), but a calendar year must have a whole number of days.

Common year

commoncommon yearsordinary year
It has a length of 365 days in an ordinary year, with 8,760 hours, 525,600 minutes, or 31,536,000 seconds; but 366 days in a leap year, with 8,784 hours, 527,040 minutes, or 31,622,400 seconds.
A common year is a calendar year with 365 days, as distinguished from a leap year, which has 366.

Leap year

leap yearsbissextile29 February
It has a length of 365 days in an ordinary year, with 8,760 hours, 525,600 minutes, or 31,536,000 seconds; but 366 days in a leap year, with 8,784 hours, 527,040 minutes, or 31,622,400 seconds.
A leap year (also known as an intercalary year or bissextile year) is a calendar year containing an additional day (or, in the case of lunisolar calendars, a month) added to keep the calendar year synchronized with the astronomical or seasonal year.

Fiscal year

FYfinancial yeartax year
Laws in many jurisdictions require company financial reports to be prepared and published on an annual basis, but generally, do not require the reporting period to align with the calendar year (1 January to 31 December).

Model year

MYmodelmodels
The model year is sometimes the same as the calendar year in which the product was manufactured, however the model year can be up to two years different from the actual year.

Summer

summer seasonaustral summersummers
In South Africa, the new school-year usually starts during the 2nd week of January, thus aligning the academic year with the Calendar year.

John Herschel

Sir John HerschelJohn Frederick William HerschelHerschel
The astronomer's mean tropical year, which is averaged over equinoxes and solstices, is currently 365.24219 days, slightly shorter than the average length of the year in most calendars, but the astronomer's value changes over time, so John Herschel's suggested correction to the Gregorian calendar may become unnecessary by the year 4000.
Herschel proposed a correction to the Gregorian calendar, making years that are multiples of 4000 not leap years, thus reducing the average length of the calendar year from 365.2425 days to 365.24225.

New Year's Day

January 1New YearNew Year’s Day
Generally speaking, a calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days.

Calendar

calendarscalendricalcalendar system
Generally speaking, a calendar year begins on the New Year's Day of the given calendar system and ends on the day before the following New Year's Day, and thus consists of a whole number of days.

Gregorian calendar

GregorianN.S.NS
The Gregorian year, which is in use in most of the world, begins on January 1 and ends on December 31.

Julian calendar

Old CalendarJulianO.S.
Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, the Julian calendar has an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days.

Hebrew calendar

Jewish calendarHebrew monthHebrew
Other formula-based calendars can have lengths which are further out of step with the solar cycle: for example, the Julian calendar has an average length of 365.25 days, and the Hebrew calendar has an average length of 365.2468 days.

Islamic calendar

AHHijriHijri calendar
The Islamic calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days.

Lunar calendar

lunarlunar yearlunar month
The Islamic calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days.

Lunar month

synodic monthsidereal monthanomalistic month
The Islamic calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days.

Season

Spring (season)

springspringtimespring season

Winter

austral wintermeteorological winterwintering

Calendar reform

reform13-month reformed calendarreform of the Calendar

ISO week date

ISO weekcalendar enumerating the weekscontains 4 January