Month Calculator

Counting months between dates is anniversary arithmetic, not division. The raw month gap is (year2 minus year1) times 12 plus (month2 minus month1); one month is then subtracted when the end day-of-month has not yet reached the start day-of-month, so 15 January to 14 March is one complete month plus 27 days, while 15 January to 15 March is exactly two. Any leftover span is expressed as days measured from the last monthly anniversary, which is why the remainder can run from 0 to 30. The decimal figure divides total days by 30.4375, the mean Gregorian month (365.25 divided by 12); the more precise constant is 30.436875 days, derived from 146,097 days per 400 years divided by 4,800 months. The unavoidable ambiguity is end-of-month behaviour. Adding one month to 31 January has no natural answer, and the two standard conventions disagree: the last-day rule clamps to 28 or 29 February, while the overflow rule rolls into early March. Financial instruments choose explicitly, and the choice changes the money. The 30/360 family used in US corporate and municipal bonds treats every month as 30 days and every year as 360, so a full year of daily accrual is 360 days rather than 365; actual/365 and actual/360 count real days on different denominators. The business-day figure here excludes Saturdays and Sundays only, with no public holidays applied, so it will overstate working time in any jurisdiction: the United States observes 11 federal holidays and England and Wales eight bank holidays.

The irregular month lengths this calculator has to accommodate are inherited from Rome. The pre-Julian Roman calendar ran ten named months from March, with the winter gap eventually filled by January and February, which is why September through December still carry the numerals for seven through ten. Intercalation was managed by the pontifices and became badly politicised, since inserting a month could extend a magistrate's term. Caesar's reform of 46 to 45 BCE fixed the lengths in essentially their present pattern; the widely repeated story that Augustus shortened February to lengthen August is a late medieval invention with no support in Roman sources. The standardised counting conventions are a product of commercial finance rather than astronomy. The 30/360 method dates back to hand-computed bond interest tables, where uniform months made accrual tractable without mechanical calculation, and it survived into modern practice as the codified variants published by the Securities Industry Association and by the International Capital Market Association, alongside the 30E/360 European form. The International Swaps and Derivatives Association fixed precise day-count and end-of-month definitions in its 1991 and 2006 definitions to remove disputes over month rolls in swap contracts. Employment and tenancy law generally went the other way, adopting the calendar-month reading, and English courts have long applied the corresponding month rule under which a period expiring on a non-existent date ends on the last day of that month.