How does adding or subtracting a date work?
The calculator applies calendar years and months first, then weeks and days. This is different from treating every month as a fixed number of days.
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For optional supporting equipment or reference material, you can browse Undated calendars and planners (paid link) on Amazon.com. Record the dates and counting choices used in your calculation. Check local holidays, time zones and official deadlines independently. No purchase is required to use this calculator.
What happens at the end of a month?
If a requested target month does not contain the original day number, the date is clamped to the last valid day of that month.
Why are weeks converted to seven days?
A calendar week is treated as seven consecutive calendar days for this arithmetic. It does not automatically skip weekends.
Can I use this for business-day scheduling?
For workday counting that skips weekends and optionally Federal holidays, use the Business Days Calculator instead.
Worked example: Add or Subtract Date
Enter the following values in the Add or Subtract Date Calculator and calculate to reproduce this result. Dates below use year-month-day order.
- Starting date: 2026-01-15
- Operation: Add
- Years: 0
- Months: 1
- Weeks: 0
- Days: 0
Example result: February 15, 2026. Adding the selected calendar amounts from January 15, 2026 gives February 15, 2026.
Calculation used: + 0y 1m 0w 0d
How to interpret this example
Adding a calendar month is not the same as adding 30 days. This example moves between matching day numbers; a starting date near the end of a month can require clamping to the last valid day of the destination month.
Frequently asked questions
Sources and methodology
This calculator uses transparent browser-side date/time arithmetic. The references below provide standards or official context relevant to the calculation.