Date Duration Calculator
Calculate days, weeks, and calendar months between two dates online. Optional IANA timezone, strict validation, and copy-friendly results.
Use YYYY-MM-DD, or English forms such as March 15, 2026 or 15 Mar 2026. Other formats are not accepted.
Date-only math uses the calendar dates you enter. Timezone selection documents how you interpret those dates and keeps behavior consistent if we extend the tool later.
How to use the date duration calculator
- Type or paste each date in the first two fields. The native calendar picker is not required; any supported text format works.
- Pick a timezone if you want to pin how calendar dates are read, or leave the default for your local zone.
- Click Calculate. Weeks show two decimal places; months use whole calendar months plus leftover days.
- Use Copy result to grab the numbers and summary, or Clear to reset everything.
How days, weeks, and months between dates are calculated
Total days are the number of calendar days from the first date to the second, not counting the start date but counting the end date. January 1 to January 31 is 30 days.
Total weeks are total days divided by 7, rounded to two decimal places for display.
Months are counted in whole calendar months from the earlier date: we add one month at a time (with day-of-month clamping) until the next step would go past the later date. The remainder is leftover days.
If the second date is earlier than the first, you still get the same span; the summary lists the earlier date first.
Date difference examples
- 2026-01-01 → 2026-01-31 — 30 days, about 4.29 weeks, 0 months and 30 days.
- 2026-01-15 → 2026-02-15 — 31 days, about 4.43 weeks, 1 month and 0 days.
- 2024-02-28 → 2024-03-01 (leap year) — 2 days across February into March.
よくある質問
Does the calculator include both the start and end date?
No. The start date is excluded and the end date is included. From the 1st to the 31st of the same month is 30 days, not 31.
How are weeks shown?
Weeks are total days divided by 7, rounded to two decimal places. That is for display; the exact day count is the source of truth.
What do “calendar months” mean here?
We add whole months from the earlier date (with day clamping for shorter months) until adding another month would pass the later date. Any extra days are listed separately.
What date formats can I use?
YYYY-MM-DD is always accepted. You can also use English month names, such as March 15, 2026 or 15 Mar 2026. Ambiguous formats like 03/04/2026 are rejected.
Do I need an account or server connection?
No. Everything runs in your browser with standard JavaScript and Intl APIs—no paid third-party date services.
Why timezone can matter for date calculations
This tool compares calendar dates only—no hours or minutes. For plain YYYY-MM-DD input, the difference is the same everywhere on the Gregorian calendar.
Choosing a timezone still matters if you want an explicit frame for how you mean each date, and it matches how browsers expose IANA zones without any paid API.