日期差计算器

How the calculation works

There is more than one sensible way to express a duration, and they give different answers:

• Total days — every 24-hour period between the two dates, including leap days
• Calendar months and years — follows the calendar, so January 31 to February 28 is one month
• Weeks — total days divided by seven

The tool shows all of them so you can pick the one that matches your question.

The leap year rule

A year is a leap year if it is divisible by 4, except centuries, which must be divisible by 400. So 2000 was a leap year, 1900 and 2100 are not. The rule has applied since 1582 and handles the Gregorian calendar exactly.

Common uses

• Age calculation — the legal age in most countries is the anniversary, not the calendar year difference
• Contract terms — "30 days from the effective date" usually excludes the start date
• Project planning — counting working days requires excluding weekends and holidays
• Countdown to a deadline — the tool shows the direction of the difference

A note on inclusive vs exclusive counting

If you count from January 1 to January 2, is that 1 day or 2? The tool uses the difference in timestamps, which gives 1. For inclusive counting — where both endpoints count — add one. Most date libraries use exclusive counting by default, which is the more common source of off-by-one errors.