Date Calculator (Add or Subtract Days)
A free date calculator that tells you the exact date a given number of days, weeks, months or years before or after any starting date, with the weekday included. Handy for payment deadlines, delivery dates and counting back from a target day, and it handles month-end rollovers correctly.
What this date calculator does
A date calculation answers the question "what date falls a given number of days before or after this one?" Counting it out on a calendar is error-prone because months vary in length (28 to 31 days) and February 29 appears only in leap years, and the drift gets worse whenever the span crosses the end of a month. This tool takes a start date and an amount and shows you the resulting date together with its weekday.
You can add or subtract in days, weeks, months or years. Days and weeks are a plain shift of the calendar, but months and years look for the corresponding day of the target month. When that day does not exist — one month after January 31, for instance — the result is clamped to the last day of that month (February 28, or 29 in a leap year) and the tool tells you so. Everything is calculated in your browser, and the dates you type are never sent to a server.
How to use the date calculator
- Set the start date Enter the day you want to count from. Today's date is filled in by default, so you can move straight on if you are working out something from today.
- Choose the direction, amount and unit Pick "After (future)" to look forward or "Before (past)" to look back, then enter the amount and the unit — days, weeks, months or years.
- Read the resulting date and weekday The result updates as you type. The weekday is shown alongside it, so you can immediately see whether a deadline lands on a weekend.
Tips for getting more out of it
- The result includes the weekday. That matters for deadlines, since a due date falling on a weekend or public holiday is often rolled forward to the next business day in practice.
- To work in weeks, choose the "week(s)" unit rather than multiplying by seven yourself — useful for pregnancy weeks or training schedules.
- Month and year calculations look for the corresponding day of the target month. If that day does not exist, the tool clamps to the last day of the month and says so.
- If you want to know how many days lie between two dates instead, use the date difference calculator, which runs the calculation in the opposite direction.
- To count only working days and skip weekends and holidays, use the business day calculator.
Frequently asked questions
Side Note — when "14 days later" is defined by law
If you have ever read a contract clause saying "within 14 days of notice" and wondered exactly where the count begins, you are in good company. Many legal systems settle the question explicitly. Japanese civil law, for example, states that when a period is defined in days, weeks, months or years, the first day is not counted. So notice received on April 1 makes April 2 day one, and the 14-day deadline lands on April 15 — unless the period happens to begin at midnight, in which case the first day does count after all.
Month-long periods get more intricate still. The same code says that periods measured in months or years follow the calendar, expiring on the day before the corresponding date in the final month. And where no corresponding date exists — one month from January 31 — the period ends on the last day of that month instead. February 28 is not an approximation someone reached for; it is what the statute prescribes.
These rules exist because a single day of drift can decide whether a payment is late or a right has lapsed. Cooling-off windows, claim periods, statutes of limitation — each of them turns on how the starting day is treated. Date arithmetic looks like simple addition, but in practice it is a field where people are right to be careful, and where seeing the weekday alongside the answer saves a second round of checking.