Add Days to Date Calculator

Adding an interval to a date behaves differently depending on the unit, and the difference is not cosmetic. Days and weeks are exact: incrementing the day number by n, or by 7n for weeks, always advances the calendar by the same amount of elapsed time. Months and years are calendar operations on named components, and they can land on a date that does not exist. Adding one month to 31 January produces 28 February, or 29 February in a leap year, because the target month is short. This end-of-month clamping is the behaviour of JavaScript's Date.setMonth, of Java's LocalDate.plusMonths, and of ISO 8601 duration arithmetic, and it is what most payment and subscription systems implement. A consequence worth understanding is that calendar arithmetic is neither commutative nor associative near month boundaries: clamping discards information, so adding one month then one day can differ from adding one day then one month. For this reason financial contracts do not rely on informal arithmetic. They specify a day-count convention such as 30/360, actual/365 or actual/actual, and a business-day roll rule such as modified following, which moves a date to the next business day unless that crosses into a new month, in which case it moves backward instead. The limitation of this calculator is that it counts calendar days only. It does not skip weekends or public holidays, so a deadline expressed in business days must be converted separately - a rough approximation is to multiply business days by seven fifths.

The practical problem of offsetting one date from another is old enough to have shaped calendar science. Medieval computus tables, used to place Easter, were essentially precomputed date arithmetic. The decisive simplification came in 1583, when Joseph Justus Scaliger proposed the Julian Period in De emendatione temporum, naming it after his father Julius Caesar Scaliger. It assigned every day a continuous integer counting from 1 January 4713 BC in the proleptic Julian calendar, so any two dates could be differenced or offset by plain subtraction and addition, with no month-length or leap-year reasoning at all. Astronomers adopted the Julian Day Number as their standard, and the Smithsonian Astrophysical Observatory introduced the Modified Julian Date, defined as JD minus 2400000.5, in 1957 for satellite tracking. Computing inherited the same trick. Unix time, dating from the early 1970s implementations at Bell Labs, counts seconds from 1 January 1970 UTC. Spreadsheet serial dates count days from an origin near the start of 1900; Lotus 1-2-3 incorrectly treated 1900 as a leap year, and Microsoft Excel reproduced the error deliberately so that file interchange would work, a bug still present today. Formal standardisation of both notation and interval semantics came with ISO 8601, first published in 1988 and now ISO 8601-1:2019, which defines durations such as P1M for one month and treats month and year offsets as calendar operations rather than fixed spans of days.