Add or subtract dates
Move a date by days, weeks, months or years.
Start with the real calendar date
Enter a starting date, choose add or subtract, then enter the number of days, weeks, months or years. The calculator returns the final date and weekday.
Use days for exact day counts. Use months or years when the task is calendar based, such as one month from a contract date or two years before an event.
Reading the new date
The result is the calendar date reached after applying the units you entered. Days and weeks are counted as fixed lengths. Months and years follow the calendar, so their length depends on the start date.
This distinction matters. Adding 30 days is not always the same as adding one month. The result should be read according to the unit you selected.
Calendar math behind the result
For days, the calculator moves forward or backward one calendar day at a time. For weeks, it converts weeks into seven day blocks. For months and years, it moves to the matching calendar month or year when possible.
If the target month does not have the same day number, the date is adjusted to a valid calendar date. This is the issue behind questions like one month after January 31.
Adding months near month-end
Starting from March 15, 2026 and adding 20 days gives April 4, 2026. Starting from March 15 and adding 3 weeks gives April 5, 2026 because 3 weeks is 21 days.
Starting from January 31 and adding one calendar month needs special handling because February has no January 31 equivalent. The calculator uses its stated month end rule.
When the date is important, rerun the calculation with days instead of months. Comparing both answers can reveal whether the unit choice matches the real problem.
Invalid dates like February 31
The biggest mistake is treating months as fixed 30 day blocks. A calendar month is not a fixed duration. It is a move to another month on the calendar.
Another mistake is mixing inclusive range counting with add or subtract math. Adding 10 days from a date means moving 10 day steps. It is not the same as counting both endpoint dates in a range.
Date Calculator FAQ
Is adding one month the same as adding 30 days?
No. Adding one month follows the calendar month. Adding 30 days follows a fixed number of day steps.
From some start dates the answers match. Around February, 31 day months and month ends, they often do not. Choose the unit that matches the wording of your problem.
What happens if I add months to January 31?
The calculator has to adjust because February does not have a 31st day. Many date tools move to the last valid day of the target month.
Always read the adjustment note when month ends are involved. This is one of the most common places where two date tools may appear to disagree.
Does subtracting days include the starting date?
No. Subtracting days moves backward by that many day steps. If you subtract 1 day from Tuesday, the result is Monday.
Inclusive counting is a different question. It is used when you count dates in a range and decide whether both endpoints are included.
If a task says before, after or from, it usually means elapsed movement. If it says through or inclusive, it usually means range counting.
Can I use this calculator for business deadlines?
You can use it for calendar math, but business deadlines may exclude weekends, holidays or local cutoff times. Those rules are not always the same as simple date addition.
If the result affects a filing, contract or official deadline, check the exact rule used by the organization.
Why do two date calculators give different month results?
They may use different month end rules. One tool may clamp to the last day of the target month. Another may roll extra days into the following month.
For clear results, use days when you need a fixed duration and months when the wording truly says calendar months.