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.

Where date calculation helps

Checking payment and settlement deadlines

Work out exactly when terms such as “30 days from the invoice date” or “month-end cut-off, paid at the end of the following month” fall, taking the date of signature or issue as your base date. The day of the week is shown too, so you can tell whether it lands on a bank holiday.

Working backwards from a delivery date

Take the delivery date as the base and count in the “before (past)” direction to derive the start date or the interim review date. It suits laying out project milestones.

Contract renewal and notice dates

Find the renewal date “one year from the start of the contract”, or the deadline for giving notice “three months before the intended termination date”, correctly, using the month-based corresponding-day calculation.

Anniversaries and event dates

Check, with the weekday shown, the date that falls a given number of days or years ahead: the hundredth day of a relationship, a first wedding anniversary, or an estimated due date (280 days from the last menstrual period).

Managing best-before dates and shelf life

Take the production date as the base and ask for “six months later” to get the best-before date. It also serves for food preparation schedules and for lot management in stock control.

Terms used in date calculation

Corresponding day: When counting a period in months or years, the day bearing the same date as the start day. The corresponding day one month after 15 January is 15 February. Where the following month has no such date — as with 31 January — the last day of that month is treated as the corresponding day.
Start day: The day from which a period begins to be counted; the “base date” in this tool. Contracts usually state it explicitly, as in “from the day the contract is concluded” or “from the day notice is received”.
Exclusion of the first day: The rule in Article 140 of Japan’s Civil Code that, where a period is fixed in days, weeks, months or years, the first day is not counted. If the period begins at midnight, however, the first day does count. Counting the day after the start day as day one is the reckoning that follows this rule.
Calendar day: A unit that counts each day on the calendar as one day, weekends and public holidays included. The unit that excludes non-working days is called a business day, and the business-day calculator handles that.
Leap year: A year in which February has 29 days. Years divisible by 4 are leap years, except that years divisible by 100 revert to common years, and years divisible by 400 are leap years once more. 2000 was a leap year; 1900 and 2100 are not.

Frequently asked questions

The direction of the calculation is reversed. The date difference calculator tells you how many days lie between two dates, while this tool tells you which date falls a given number of days before or after one date. Use the former when you know both dates, and this one when you know the interval.

No. The month unit looks for the corresponding day in the target month, so one month after January 15 is February 15 (31 days later), while one month after April 15 is May 15 (30 days later). If you specifically want 30 days, choose the "day(s)" unit and enter 30.

February has no 31st, so the result is clamped to the last day of February (the 28th, or the 29th in a leap year) and a note explains the adjustment. This matches how month-end dates are normally treated in contracts.

This tool counts calendar days, including weekends and holidays. For counting in business days instead, use the business day calculator, which also lets you choose whether Saturdays count as working days.

Yes. Choose "Before (past)" as the direction to count backwards from the start date — useful for working back from a delivery date to a kick-off date, or for checking a notice deadline before a cancellation.

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.

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