Date Difference Calculator

There are two legitimate answers to the question of how far apart two dates are, and this calculator reports both. The elapsed count treats each date as milliseconds since the Unix epoch of 1 January 1970, subtracts, and divides by 86,400,000 to give whole days. The calendar breakdown instead subtracts the year, month and day components separately, borrowing from the preceding month when the day component goes negative. These disagree by design: the interval from 31 January to 1 March is 29 elapsed days but reads as one month and one day, and dividing a day total by 365.25 will not reproduce a year-month-day breakdown. Business days are counted by iterating the interval and skipping Saturdays and Sundays; across whole weeks this is exactly five sevenths, or 71.4 percent. Inclusive versus exclusive counting is the commonest source of disagreement. A duration - how long something lasted - normally excludes the end date, so 1 January to 2 January is one day. A count of days covered, such as a hotel stay or a seven-day medication course, includes both endpoints and gives two. Court rules add a third convention, clear days, excluding both the first and last day. Two technical limits apply. Public holidays are not removed, so the business-day figure overstates real working days by up to 11 in a US calendar year. And the hour, minute and second totals are days multiplied by 24, which is nominal rather than physical: where daylight saving applies, one civil day each year runs 23 hours and another 25.

Counting days between dates became commercially critical long before it became easy. Because interest accrues by time, lenders needed a reproducible convention, and the 30/360 method - treating every month as 30 days and every year as 360 - was adopted precisely because it made manual interest calculation tractable, dividing evenly and avoiding month-length reasoning. It remains standard in parts of the US corporate and municipal bond markets, and was formalised for that market in Standard Securities Calculation Methods, published by the Securities Industry Association in 1973. Competing conventions - actual/360, actual/365 and actual/actual - are now defined precisely in the ISDA 2006 Definitions for derivatives. Law developed its own reckoning rules. Roman-derived civil computation and the English maxim dies a quo non computatur in termino, the day from which time runs is not counted, produced the exclusive-counting default that survives in modern procedural rules. Calendar history complicates historical differencing: an interval spanning September 1752 in Britain and its colonies must account for the eleven days from the 3rd to the 13th that were removed when the Gregorian reform of 1582 was finally adopted, and intervals crossing 1582 elsewhere in Europe face the same problem ten days wide. The computational basis used here is younger. Representing time as milliseconds since 1970 was standardised for the web in ECMA-262, first published in 1997, which is what makes exact elapsed-day differencing possible in a browser.