Date calculators

Every calculator below does one of two things: it starts from a date and moves forward or backward by a fixed span — a set number of days, weeks or months — to land on a new date, or it starts from two existing dates and measures the span between them. The first kind answers "what date is 90 days from now"; the second answers "how long is it between these two dates". They look similar because both involve a calendar, but the arithmetic behind each is different, and the second kind in particular has more than one defensible answer depending on what "how long" is actually being asked.

Adding a month is not the same as adding 30 days

A week is always seven days and a day is always 24 hours, so adding days or weeks to a date is pure arithmetic with no ambiguity. A month is not a fixed length — it runs anywhere from 28 to 31 days depending which one it is and whether it is a leap year — so "add one month" has to be defined rather than just calculated. The usual convention, and the one used throughout this section, is to keep the day-of-month the same and move the month forward, clamping to the last valid day when the target month is too short: 31 January plus one month lands on 28 February in a normal year, or 29 February in a leap year, not on 3 March, which is what adding 30 days literally instead would give. The two methods agree most of the time and disagree specifically around month-end dates, which is exactly when the difference tends to matter — a payment due date, a notice period, or a deadline set as "one month from" a date near the end of a month.

Leap years, and why 29 February is a special case twice over

A year is a leap year if it divides evenly by 4, except century years, which are leap years only if they also divide by 400 — 2000 was a leap year, 1900 was not, and 2100 will not be either. That rule affects every calculator on this page that spans more than a few weeks, because a "days from date" or "years between two dates" result that crosses a 29 February will differ by a day from an otherwise identical calculation that does not. It also creates a genuinely unresolved edge case for anyone born on 29 February: in a non-leap year they have no literal birthday to count from, and different institutions handle it differently — most treat the following day, 1 March, as the legal birthday, though a minority use 28 February instead. The age and years-between-two-dates calculators here follow the more common 1 March convention.

Two honest answers to "how long between these dates"

Age and duration calculators can report a span two different ways, and both are correct — they are just answering different questions. A calendar-based answer breaks the gap into whole years, months and days, the way people actually describe their own age: "34 years, 7 months and 2 days". A total-elapsed answer instead gives a single running count — total days, or total weeks — which is more useful for anything measured against a fixed threshold, such as an eligibility rule written as "must be at least 18,250 days old" or a notice period specified in total days rather than in calendar months. The two do not convert into each other by simple division, because the calendar-based figure is uneven — months and years vary in length — while the total is exact, which is why both versions are offered separately rather than one being derived from the other after the fact.