Datetime Duration Calculator
Enter an ISO datetime with an offset, choose a direction and unit, then calculate the resulting instant locally.
Enter an offset-aware datetime and duration.
Result
Month-end adjustment applied.
Calendar months are variable length; elapsed days are exactly 24 hours.
A QUICK WALKTHROUGH
How to use this tool
- Enter an ISO datetime ending in Z or containing a numeric offset.
- Choose add or subtract, a whole amount, and a unit.
- Review the ISO result and any month-end adjustment note.
Calendar and elapsed units
Years and months change calendar fields. Weeks, days, hours, minutes, and seconds use elapsed UTC time; one day is exactly 24 hours.
Offset and month-end rules
The input must include Z or a numeric offset. Calendar arithmetic is applied in the input offset, and an unavailable day is clamped to the target month’s last day.
Strict bounds
Amounts are whole numbers from 0 through 1,000,000. Results must remain within years 0001 through 9999.
GOOD TO KNOW
Common questions
Is a month a fixed number of days?
No. Months follow calendar rules and vary in length.
Can I subtract a duration?
Yes. Select Subtract.
Are values sent to a server?
No. The calculation runs locally in the browser.