How it works
Choose a mode. Days between dates counts the time from one date to another; Add or subtract moves a date forward or back by days, weeks, months or years. Everything is calculated on your device with the Gregorian calendar, including leap years.
Days = (later date − earlier date) ÷ 86,400,000 ms
The two modes
- Days between dates: the exact number of days, plus weeks, calendar years/months/days, weekdays and weekend days.
- Add or subtract: enter a negative amount to count backwards. Month arithmetic clamps to the last valid day, so 31 January plus one month is 28 February.
- Include the last day: use it when a period is counted inclusively, such as hotel nights or contract terms.
Tips
- Dates before 1970 and far-future dates work fine — the calculation is calendar-based, not timestamp-based.
- Weekdays are counted Monday to Friday; public holidays are not excluded because they differ by country.
- D-day counters usually count up to the day itself, so tick “Include the last day”.
Standards and sources
- Gregorian calendar with leap-year rule (divisible by 4, except centuries not divisible by 400)
- Day arithmetic follows ISO 8601 date notation (YYYY-MM-DD)
Last reviewed October 7, 2026
Frequently asked questions
How many days are between two dates?
Enter both dates and the calculator counts the difference in days. Tick “Include the last day” if you need an inclusive count, which adds one day.
Does it count the last day?
Only if you tick the box. Counting the last day is standard for hotel nights, notice periods and D-day counters, where the final day is part of the period.
How are months and years added?
Months and years are added on the calendar, not by adding 30 days. If the target day does not exist — 31 January plus one month, for example — the result is clamped to the last day of that month.
Are leap years handled?
Yes. The calculation follows the Gregorian calendar, so 29 February is counted correctly and the 100-year rule is respected.
Are public holidays excluded from weekdays?
No. Public holidays differ by country and change every year, so the weekday count covers Monday to Friday only.