About the Date Calculator
This date calculator does two things. ① Add or subtract time finds the date that is a given number of days, weeks, months or years before or after a date: a deadline, a 90-day return window, the end of a notice period or the date 100 days from today. ② Days between two dates counts how far apart two dates are, in days, weeks, months and years, and how many of those days are weekdays and weekend days. Results update as you type.
How to use
- In ①, enter a start date and an amount (number + days, weeks, months or years), then choose Add or Subtract.
- If the start date itself counts as the first day (e.g. a 30-day trial that starts today), tick Count the start date as day 1.
- In ②, enter a start and an end date to see the number of days between them, plus weeks, months and weekdays.
- To count both the first and the last day (e.g. days of a trip or a hospital stay), tick Include the end date.
Including or excluding the start date
100 days after October 1, 2026 is January 9, 2027. If October 1 counts as day 1, the 100th day is one day earlier: January 8, 2027.
| Example | How to count |
|---|---|
| Delivery in 3 days | Start date not counted |
| “Day 100” of a challenge or a new baby | Start date = day 1 |
| 1-year contract starting Oct 1 (ends Sep 30) | Start date = day 1 |
| Nights in a hotel | Days between, end date not included |
How months are added
Months and years are added on the calendar: the same day number in the target month. If that day doesn’t exist, for example one month after January 31, the result is the last day of the month (February 28, or 29 in a leap year). “Months + days” between two dates is counted the same way: full calendar months first, then the remaining days.
FAQ
Does the day count include the start date?
By default, the calculator counts like a subtraction (end date − start date), so the start date is not included. Tick “Include the end date” to count both ends, which adds one day.
Are public holidays excluded from weekdays?
No. Weekdays are Monday to Friday; only Saturdays and Sundays are excluded. Subtract any public holidays in the period yourself.
How far back or forward can I calculate?
From year 1 to year 9999. Dates before October 1582 use the current Gregorian calendar projected backwards, so they may differ from historical Julian-calendar dates.
How do I calculate how many weeks pregnant I am?
Enter the first day of your last period as the start date and today as the end date; “Weeks + days” is the gestational age. Your doctor or midwife will confirm the exact due date.