If you just need the answer, the time duration calculator gives it instantly, times of day included. This guide shows how the counting actually works, so you can check a result or do it without a computer.
The method: count to the end of each month
- Days left in the starting month: that month's length minus the start day.
- Full months in between: add each one's length.
- The ending month: add the end day's number.
Example: March 1 to April 15, 2025
30 + 15 = 45 days
Month lengths and leap years
April, June, September, and November have 30 days. February has 28 — or 29 in a leap year. Every other month has 31.
A year is a leap year if it divides by 4, except century years, which must also divide by 400. So 2024 and 2028 are leap years, 2000 was one, and 1900 was not. Getting this wrong is the most common mistake in hand counting, and it only bites when your span includes the end of February.
January 15 to March 10 is 55 days in 2024 (16 + 29 + 10) but 54 days in 2025 (16 + 28 + 10).
Inclusive or exclusive?
The method above counts the gap between two dates, which is what "how long until" usually means. If both the first and last day count — a three-day conference running March 1 to March 3, say — add one. March 1 to April 15 is 45 days apart, but 46 days if you count both ends.
Crossing into a new year
Nothing changes; December just has 31 days. December 20, 2025 to January 5, 2026 is 31 − 20 = 11 days left in December, plus 5, for 16 days.
Turning days into weeks and hours
Divide by 7 for weeks: 45 days is 6 weeks and 3 days. Multiply by 24 for hours: 45 days is 1,080 hours. One caveat — if the span crosses a daylight-saving change, the real elapsed time is an hour more or less than days × 24.
Where this comes up
- Deadlines: days until a project is due
- Events: how long until a wedding, trip, or conference
- Ages: exact age in years, months, and days
- Contracts: time left on an agreement or subscription
For spans that include times of day, or when you'd rather not count, the time duration calculator gives a calendar-accurate breakdown. To watch the days tick down to a future date, use the countdown timer.