How to use this calculator
The add or subtract date calculator moves a calendar date by years, months, weeks, and days. Enter your starting date and the quantities to add. Use negative quantities to move backward. It is useful for planning follow-up dates, comparing month-based intervals, or checking where a schedule lands after several calendar adjustments.
Choose how to handle a day that does not exist in the target month. Constrain uses the last valid day of that month; reject reports the overflow instead. The calculator combines years and months before applying the combined weeks and days. That sequence is visible in the breakdown, so mixed-unit calculations can be reviewed step by step.
Formula and calculation method
Target date = start + (years × 12 + months) calendar months, then + (weeks × 7 + days) calendar days.
Apply calendar units in a defined order
A year-and-month offset first selects a target month. If the original day is unavailable there, the selected overflow policy resolves the problem. The week-and-day offset is applied afterward. Mixed units are not interchangeable with a fixed number of seconds, and changing the order can produce a different answer near month boundaries.
Choose a month-end policy
Constrain is useful when an interval should land on the final available date, such as moving from the 31st into a shorter month. Reject is useful when you need to know that the exact day cannot be preserved. Neither choice defines a recurring billing or legal deadline policy; it simply makes the arithmetic rule explicit.
Worked examples
One month after January 31
January 31, 2024 plus one month lands in February, which has 29 days that year. Constrain therefore returns February 29. Reject instead identifies the missing day. In a non-leap year, constrain would return February 28. The result is a calendar month offset, not a fixed 30-day duration.
Add weeks and days
Starting on April 1, 2024, add two weeks and three days. Two weeks contributes 14 days, and the additional three makes 17. The target is April 18. This type of offset is independent of the lengths of intervening months because the combined quantity is expressed entirely in days.
Move backward from a month end
March 31, 2024 minus one month constrains to February 29. Adding one month to February 29 gives March 29, not March 31. The earlier constrain step discarded the original day number. Keep the original anchor date if you intend to generate a recurring series that always uses month end.
Edge cases and practical checks
Leap-day anniversaries
Adding a year to February 29 needs an overflow policy if the destination year is not a leap year. Review whether your application expects February 28 or another rule. The calculator offers constrain or reject; it does not silently apply a separate anniversary convention such as always moving to March 1.
Working-day and timestamp boundaries
All days count here, including weekends and holidays. Use Business Days Calculator to skip nonworking dates. Dates are limited to 1900–2100, quantities must be bounded whole numbers, and out-of-range results are rejected. A date output also has no time of day, so it cannot by itself specify a global event instant.
Frequently asked questions
How do I subtract days from a date?
Enter a negative value in the days field. For example, minus ten moves the starting date ten calendar steps backward. Leave the other offsets at zero when you only want a day subtraction, and check the resulting date in the calculation breakdown.
Is one month always 30 days?
No. A month offset moves to another calendar month, whose length can be 28, 29, 30, or 31 days. Adding 30 days is a different operation. Choose the unit that matches your scheduling rule instead of treating the two operations as substitutes.
What happens if the target day does not exist?
Constrain returns the last valid day in the destination month. Reject reports an error instead of producing a date. January 31 plus one month is the familiar example: February does not have a 31st, so a policy is necessary.
Can I combine positive and negative quantities?
Yes. Each field can have its own sign. Years and months are combined first, and weeks and days afterward. Inspect the breakdown when mixing signs so you can see the net calendar-month move and the following day adjustment separately.
Why does subtracting the same amount not restore my date?
Month-end constraining can lose information. Moving from the 31st into February changes the day to February’s final valid day. Reversing the month offset starts from that new day and cannot infer the original 31st. Preserve an anchor for recurring month-end schedules.
Does this skip public holidays?
No. This calculator moves through the calendar without a working-day schedule. For holiday-aware planning, use Business Days Calculator and enter the closure dates yourself. Always check that the closure list covers the entire period you intend to calculate.
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