Date & Time

Add or Subtract Days from a Date

Shifts a whole list of dates forward or back, and says where a month end had to be clamped.

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How to use this tool

  1. Paste your starting dates, one per line.
  2. Choose add or subtract, the amount, and the unit.
  3. Select Calculate.
  4. Check the Note column for month-end clamping, and the Weekday column for anything that landed on a weekend.

What add or subtract time does

Adding days is arithmetic. Adding months is a convention, because 31 January plus one month has no obvious answer — there is no 31 February. Every database clamps to the last day of the target month, so this page does the same and marks each row where clamping happened, rather than producing a quietly different date and saying nothing.

The weekday column is there because most of the reasons for shifting a date end in a question about working days. A due date thirty days out that lands on a Sunday is a due date you will want to move, and seeing that at the same time as the date saves a second lookup.

Frequently asked questions

You get 28 February, or 29 in a leap year, and the row is marked as clamped. There is no 31 February, so every system has to pick a convention; clamping to the last day of the target month is what databases and date libraries do, and it is the least surprising of the options.

No, and the difference is a recurring source of billing bugs. Four weeks is always 28 days; a month is 28 to 31 and drifts against the weekday. If a schedule needs to fall on the same weekday, use weeks; if it needs to fall on the same date, use months.

Because a date thirty days out is usually a deadline, and a deadline on a Sunday is a deadline that will be missed. Seeing the weekday next to the result saves the second lookup that everybody does anyway.