Can I find a start time from an end time and duration?

Not directly — enter the known end time as the start and work with the difference mode, or subtract manually.

Does this account for time zones?

No — it works with the plain time-of-day values you enter, with no time zone conversion involved.

Can I add more than 24 hours?

Yes — entering more hours than fit in a day wraps around correctly to the resulting time, though the tool shows only the time of day, not how many days later it falls.

Why does the difference always show a positive duration?

It wraps forward to the next occurrence of the end time, so an end time earlier than the start (like 11 PM to 2 AM) is treated as spanning into the next day rather than showing a negative result.

All tools

Math & Everyday

Time Duration Calculator

Add a duration to a time, or find the gap between two.

🔒 Runs entirely in your browser — nothing here is ever uploaded

About the Time Duration Calculator

Adds a duration to a starting time to find an end time, or calculates the gap between two times of day.

How to use it

To add a duration: enter a Start time, then Hours and Minutes to add.
To find a gap: enter a Start and End time in the second section.
Read the result in each section's readout.

Formula

End time = (Start time + hours×60 + minutes) mod 1440 minutes
Duration = (End time − Start time) mod 1440 minutes

Worked example

Starting at 9:00 AM and adding 2 hours 30 minutes gives 11:30 AM.

Recommendations

• Times wrap around midnight automatically — adding hours past 24:00 rolls into the next day's time.
• This tool only tracks time-of-day, not date — use the Date Calculator for spans across multiple days.
• The difference calculator always returns a positive duration by wrapping forward to the next occurrence of the end time.

Frequently asked questions

Yes — the difference calculation wraps around midnight automatically, so an 11 PM to 2 AM span correctly shows as 3 hours.

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