A calendar year begins on New Year's Day and ends the day before the next, consisting of a whole number of days. The widely used Gregorian calendar year starts on January 1 and ends on December 31, lasting 365 days in a common year and 366 days in a leap year, averaging 365.2425 days. Other calendars differ: the Julian calendar averages 365.25 days, and the Hebrew calendar about 365.2468 days. The Islamic calendar is lunar-based with 354 or 355 days. The astronomer's mean tropical year is 365.24219 days, slightly shorter than most calendar years.
Quarter year
"Fourth quarter" redirects here. For the sports term, see Comeback (sports).
The calendar year can be divided into four quarters,4 often abbreviated as Q1, Q2, Q3, and Q4. Since they are three months each, they are also called trimesters. In the Gregorian calendar:
- First quarter, Q1: January 1 – March 31 (90 days or 91 days in leap years)5
- Second quarter, Q2: April 1 – June 30 (91 days)
- Third quarter, Q3: July 1 – September 30 (92 days)
- Fourth quarter, Q4: October 1 – December 31 (92 days)
In some domains, weeks are preferred over months for scheduling and reporting, so they use quarters of exactly 13 weeks each, often following ISO week date conventions. One in five to six years has a 53rd week which is usually appended to the last quarter. It is then 98 days instead of 91 days long, which complicates comparisons.
In the Chinese calendar, the quarters are traditionally associated with the 4 seasons of the year:
- Spring: 1st to 3rd month
- Summer: 4th to 6th month
- Autumn: 7th to 9th month
- Winter: 10th to 12th month
Quadrimester
The calendar year can also be divided into quadrimesters (from French quadrimestre),6 lasting for four months each. They can also be called the early, middle, or late parts of the year. In the Gregorian calendar:
- First quadrimester, early year: January 1 – April 30 (120 days or 121 days in leap years)
- Second quadrimester, mid-year: May 1 – August 31 (122 days)
- Third quadrimester, late year: September 1 – December 31 (121 days)
Semester
The calendar year can also be divided into semesters,7 lasting six months each and often being abbreviated as S1 and S2. In the Gregorian calendar:
- First semester, S1: January 1 – June 30 (181 days or 182 days in leap years)
- Second semester, S2: July 1 – December 31 (184 days)
See also
- Academic term – Subdivision of the academic year at educational institutions
- Bissextus – Leap day (24 or 29 February) (historical usage)
- Calendar reform – Significant revision of a calendar system
- Common year – Calendar year with 365 days
- Fiscal year – One-year term for government and business financial reporting
- Intercalation (timekeeping) – Insertion of a leap day, week, or month
- ISO 8601 – International standards for dates and times
- ISO week date – Leap week calendar system
- Julian date – Days since the beginning of the Julian PeriodPages displaying short descriptions of redirect targets
- Julian year (astronomy) – a time interval of exactly 365.25 Earth days
- Julian year (calendar) – a year in the Julian calendar that is either 365 or 366 days, or 365.25 days on average
- Leap year – Calendar year containing an additional day
- Model year – Production date of a commercial product
- Ordinal date – Date written as number of days since first day of year
- Seasonal year – Time between successive recurrences of a seasonal event
Notes
References
"calendar year". Cambridge Dictionary. Archived from the original on 2022-05-03. Retrieved 2025-01-12. http://web.archive.org/web/20220503102342/http://dictionary.cambridge.org/dictionary/english/calendar-year ↩
The other Islamic calendar, observed in Iran, is the Solar Hijri calendar. It runs from spring equinox to spring equinox. /wiki/Solar_Hijri_calendar ↩
"calendar year". Merriam-Webster. Retrieved 6 August 2014. http://www.merriam-webster.com/dictionary/calendar%20year ↩
"Calendar quarter". Merriam-Webster (Merriam-Webster.com). Retrieved 23 December 2024. https://www.merriam-webster.com/dictionary/calendar%20quarter ↩
"Calendar quarter". Cambridge Business English Dictionary (Dictionary.Cambridge.org). Retrieved 23 December 2024. https://dictionary.cambridge.org/us/dictionary/english/calendar-quarter ↩
"QUADRIMESTRE : Définition de QUADRIMESTRE". Centre National de Ressources Textuelles et Lexicales (cnrtl.fr). Retrieved 4 January 2025. https://www.cnrtl.fr/definition/quadrimestre/ ↩
"Semester". Merriam-Webster (Merriam-Webster.com). Retrieved 4 January 2025. https://www.merriam-webster.com/dictionary/semester ↩