Add Time Calculator

Adding time across midnight, month ends and leap days is fiddly by hand. Use it to find when a flight lands, a long task finishes or a medication dose is due. Daylight saving changes are not applied – add or subtract an hour yourself if one falls in between.

How to use the Add Time Calculator

1. Start date
2. Start time
3. Days to add
4. Hours to add
5. Minutes to add
6. Seconds to add

Results update as you type, and every field accepts the units shown in its menu.

How it works

Enter the start date and time, then the days, hours, minutes and seconds to add.

Tips

• Check whether the end date should be included – event durations usually include it, elapsed time does not.
• Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
• Use 24-hour time or set AM/PM carefully when entering clock times.
• Time zones and daylight-saving changes are not applied unless the calculator says so.

Common mistakes to avoid

• Counting a 30-day month when a calendar month is meant (or the reverse).
• Forgetting leap years when counting days across February.
• Mixing decimal hours (7.5 h) with clock time (7:30).

Inputs explained

What each result means

• Date = floor([Result (days since epoch)] + 0.0000001)
• Time = ([Result (days since epoch)] – [Date])×24
• Time (24-hour) = ([Result (days since epoch)] – [Date])×24
• Weekday = weekday([Date])
• Total time added = ([Days to add]×24 + [Hours to add] + [Minutes to add]÷60 + [Seconds to add]÷3600)