Time & Date Calculators

Date arithmetic is deceptively awkward because the units are irregular. Months have different lengths, years occasionally have an extra day, and adding one month to 31 January has no obviously correct answer. These tools apply consistent, stated rules so the result is reproducible rather than surprising.

The rules used here are the conventional ones. Adding days or weeks counts calendar days including weekends and holidays. Adding months keeps the same day number where the target month has one and clamps to the last day where it does not, so 31 January plus one month is 28 or 29 February. Leap years follow the full Gregorian rule: divisible by four, except centuries, unless divisible by 400 — which is why 2000 was a leap year and 1900 was not.

Live tools read your device's own clock and time zone, so the answer is local to you rather than to a server. That matters most around midnight and around daylight-saving changes, where the same instant is on two different dates depending on where you are standing. Week numbers follow ISO-8601, where weeks start on Monday and week 1 is the week containing the first Thursday of the year.