Deadline Arithmetic Reference

General information, not legal advice.This page publishes no jurisdiction’s limitation periods, notice lengths or return windows, because those vary by place and by claim type and a single published set would be wrong for most readers. Every period on this site is one you supply. What is shown here is how the calendar arithmetic behaves once you know your own period — and which period applies to you is a question for a professional where you live.

Month ends clamp to the last valid day

There is no 31st of February, so adding a month to a date near the end of a long month lands on the last day of the shorter one. The same rule handles the leap day: a 29 February start plus twelve months falls on 28 February in a year that has no 29th.

Two start dates can share one deadline

Because of that clamping, distinct start dates collapse onto the same answer. Adding one month to 2026-01-29 or 2026-01-30 or 2026-01-31 produces an identical date.

Working forwards this is harmless. Working backwards from a known deadline to reconstruct when a clock started, it means the answer may not be unique — so establish a start date from the record rather than by reversing the arithmetic.

One month is not thirty days

A month’s span moves with the calendar — as few as 28 days from a start on 2026-01-31. Four weeks is always twenty-eight days. Thirty days is always thirty. If your clause says months, count months; if it says days, count days, and do not substitute one for the other because they look close.