Add or Subtract Days From a Date

A surprising number of hand-rolled date calculations — a spreadsheet formula, a quick script — add days by simply adding a fixed number of seconds or by naively incrementing a day-of-month field without checking whether the result rolled past the end of that month, both of which silently produce a wrong date around month boundaries and especially around February. Correctly adding a duration means genuinely walking the calendar forward: recognising that adding a month to 15 January gives 15 February, that adding a month to 31 January lands on the last day of February rather than overflowing into March, and that a leap year changes which last day that is. This is exactly the class of edge case that hand-rolled date arithmetic gets subtly wrong most often, and it is precisely what this tool is built to handle correctly.

Everything about adding a duration applies symmetrically to subtracting one — a negative number of days, weeks, months or years walks the calendar backward using the identical month-length and leap-year-aware logic, which matters for exactly the same reason it matters going forward: working out when a notice period must have started to expire by a given date, or confirming the date something must have happened by counting backward a fixed interval from a known deadline.