Month to Year (365 days)

The Roman calendar originally had 10 months (March through December), leaving winter uncounted. January and February were added by King Numa Pompilius around 713 BCE to cover winter. To keep the total at 355 days (a lunar year), February got the leftover days — 28. Julius Caesar's 365-day calendar reform kept February shortest. The "30 days hath September" mnemonic reflects decisions made by Roman senators to honor Augustus and Julius Caesar with 31-day months, robbing days from February.

Four weeks are exactly 28 days. The selected month in this converter is 365/12 days, approximately 30.4167 days, so it is not four weeks. Actual Gregorian calendar months have 28–31 days; February in a common year has exactly four weeks. Use calendar dates when the actual month length matters.

The irregular distribution (JMMJSND = 31 days; AJJN = 30 days; Feb = 28/29) comes from Julius Caesar's calendar reform (45 BCE) and later Augustan adjustments. Caesar gave odd months 31 days and even months 30 (with February 29/30). Augustus then renamed "Sextilis" after himself and extended it to 31 days to match Julius's month — taking a day from February. The resulting pattern has persisted for 2,000 years.

A sidereal month is the time for the Moon to orbit Earth relative to distant stars: 27.32 days. A synodic month is the time between identical lunar phases (new moon to new moon): 29.53 days. The synodic month is longer because Earth moves along its orbit — the Moon must travel further to reach the same phase angle relative to the Sun. Islamic and Hebrew lunar calendars are based on the 29.53-day synodic month.

Yes. The converter defines a month as exactly 2,628,000 seconds and a year as exactly 31,536,000 seconds, so 12 months equal one 365-day year. This is a fixed-duration convention, not a lunar-month average or a calendar calculation that accounts for leap days.

No. The selected year always represents exactly 365 days. A Gregorian leap year has 366 days, and an interval between calendar dates may include leap days. Use date-aware calendar arithmetic for those intervals rather than treating this fixed-duration year as a calendar engine.

Pope Gregory XIII corrected the Julian calendar, which had drifted 10 days from the astronomical equinox since 325 CE. Catholic countries deleted 10 days: Thursday 4 October 1582 was followed by Friday 15 October. Protestant countries delayed adoption — Britain switched in 1752, deleting 11 days (2 September → 14 September). Russia switched in 1918, deleting 13 days. This is why the "October Revolution" occurred in November under the new calendar.

A fiscal year (FY) is a 12-month accounting period that may start on any date. The US federal government uses 1 October – 30 September. The UK tax year runs 6 April – 5 April (a date inherited from the Julian-to-Gregorian calendar switch: the old new year was 25 March, which shifted to 5 April after the 11-day deletion). Australia's FY is 1 July – 30 June. Companies choose fiscal years to align with seasonal revenue patterns.

The universe is approximately 13.787 billion years old, determined from measurements of the cosmic microwave background (Planck satellite data, 2018). The oldest known star (HD 140283, 'Methuselah star') is about 14.46 ± 0.80 billion years — uncertainties in stellar age models make this compatible with the universe's age. Earth is 4.54 billion years old. Modern humans (Homo sapiens sapiens) have existed for about 300,000 years — 0.0022% of Earth's age.

Ethiopia uses the Ethiopian calendar, which has 13 months (12 of 30 days + a 13th month of 5 or 6 days). It is currently about 7–8 years behind the Gregorian calendar because Ethiopian Christianity followed a different calculation for the year of Jesus's birth. Ethiopia celebrated its millennium (year 2000) in September 2007. This is why Ethiopian Airlines advertises '13 months of sunshine' — their calendar has a literal 13th month.