Add or Subtract Days
Add or take away days, weeks, months or years from a date.
Add subtract days moves a date forward or backward by an amount you choose. Give it a starting date and a number of days, weeks, months or years, and it returns the date you land on.
How it counts
Days, weeks, months and years are added by the calendar. Adding a month to 31 March gives 30 April, because April has no 31st — the same rule every calendar uses.
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How to use Add subtract days
Enter the starting date. Choose whether to add or subtract. Enter the amount — days, weeks, months or years. Read the resulting date, along with the weekday it falls on. The weekday matters more than people expect. A date thirty days out is useless information if it turns out to be a Sunday and the office you need is closed.
Adding months is not adding days
Adding days is simple arithmetic. Adding months is a rule, and the rule has an edge case. Add one month to 31 January and there is no 31 February to land on, so the result clamps to the last day of the month — 28th, or 29th in a leap year.
This has a consequence worth knowing: adding a month and then subtracting a month does not always return you to where you started. Once a date has been clamped to the end of a short month, the original day number is gone. If the exact day is important, work in days, where nothing is ever clamped.
Add one month to 31 January, subtract one month, and you get 28 January. The calendar is not sorry.
What this is used for
Deadlines — "within 30 days of receipt" turned into an actual date on the calendar. Notice periods — counting forward from the day notice was given. Expiry and renewal — documents, warranties, subscriptions, prescriptions.
Working backwards — subtracting from a fixed event date to find when preparation must start. Reminders — the date to check on something you have just sent away.
Forward and backward are not mirror images
Subtracting behaves the same way as adding, with the same month-end clamping — and the same asymmetry. If your calculation has to be reversible, or has to be checked by someone else, state it in days. A span in days is unambiguous, survives the trip across a month boundary, and produces the same answer no matter who runs it.
Where a legal or contractual period is written in months, use months, because that is what the wording means. Where it is written in days, use days. The mistake is translating between the two on the assumption that a month is thirty days — it is only sometimes, and the exceptions are the dates people argue over.
Questions and answers
- What happens when the result has no such date, like 31 February?
- It clamps to the last valid day of that month — 28 or 29 February, depending on the year.
- Does the count include the starting date?
- No. Adding one day to a Monday gives Tuesday. The start date is the origin, not the first day counted.
- Does it skip weekends?
- No, every calendar day is counted. To move forward by business days only, use a working-days count instead.