Days Between Dates Calculator
＋More features: breakdown in weeks and months, and the date N months later
Breakdown in weeks and months
Shows the difference between the start and end dates above also as weeks plus remaining days and as an approximate number of months (counting the first day gives the style used for hospital-stay counts and the like).
Date N months later (or earlier)
Gives the date and weekday the specified number of months after the base date above (negative for earlier).
Instantly see how many days lie between two dates. You can also calculate a future date, such as “what date is 100 days from now?”. Useful for counting down to an event or for period calculations on paperwork.
How to use
- Days between dates: pick a start date and an end date — the number of days in between appears the moment you choose them. No button needed.
- Date N days later: pick a base date and enter a number of days (negative for the past); the resulting date and weekday appear automatically.
How it is calculated
- Based on the Gregorian calendar (the current calendar), dates are converted to serial day numbers to compute differences and additions. Leap years are handled (one every 4 years, except that years divisible by 100 but not by 400 are common years).
- Elapsed days do not include the first day (July 18 → July 19 is “1 day”). Days including both ends add 1 to this.
- Example: 2026-07-18 → 2026-08-01 is 14 days (15 days including both ends)
Sources and assumptions
- Arithmetic based on the Gregorian calendar (leap-year rules).
- Public holidays and business days are not taken into account (the calculation is calendar-day based).
Assumptions and notes
- To count in business days (excluding weekends and holidays), subtract the days off from this tool's result.
- Legal period calculations (such as the first-day-exclusion principle of the Japanese Civil Code) follow their own specific rules. For important deadlines, check the governing provisions.
Frequently asked questions
Does “N days later” include the day itself?
No. One day after July 18 is July 19. If you want to count the day itself as day 1, subtract 1 from the number of days you enter.
Can it calculate months or years ahead?
This tool works in days only. Because months have different lengths (28–31 days), convert to days first (as a guide: 1 month ≈ 30 days, 1 year = 365 or 366 days).
Can I use past dates?
Years 1600 to 2500 are supported. Enter a negative number of days to get the date N days before.