Time Duration Calculator

What this tool is used for

• Working out billable hours between a start and an end time without counting on your fingers.
• Finding the gap between two timestamps in a log to measure how long something took.
• Calculating a shift length that crosses midnight.
• Checking the number of days between two dates for a notice period or a deadline.
• Summing several intervals to check a timesheet total.

Frequently Asked Questions

How do I subtract times crossing midnight?

Add 24 hours to the end time when it is earlier than the start. A shift from 22:00 to 06:00 is 8 hours, not −16 — naive subtraction gives a negative that silently propagates into payroll and scheduling.

How do I convert the result to decimal hours?

Divide the minutes by 60, not by 100. 7 h 30 m is 7.5 hours, but 7 h 45 m is 7.75, not 7.45. Treating minutes as hundredths is the classic timesheet error and always underpays.

Does this account for daylight saving?

A plain clock-time difference does not — on a DST transition day the elapsed real time differs from the clock difference by an hour. For durations that must survive transitions, work in UTC and convert only for display.

How are breaks handled?

As a separate subtraction from the gross duration. Whether an unpaid break is deducted is a policy question, not an arithmetic one, and jurisdictions differ on when breaks must be paid.

Why do my hours not sum correctly?

Usually rounding applied per entry rather than once at the end. Rounding each day to the nearest quarter-hour and then summing can drift materially over a month against summing first and rounding once.

How should durations be stored?

As a single unit — seconds or minutes — rather than as hours and minutes fields. Two fields invite invalid states such as 90 minutes, and every arithmetic operation has to carry between them.

Why do rounded timesheet entries not add up?

Because each entry is rounded before summing, so the errors accumulate rather than cancel. Summing exact durations and rounding once at the end is what makes a weekly total match the daily figures.

Common errors and gotchas

• Ignoring a daylight-saving transition, which makes one day 23 or 25 hours long.
• Treating a duration crossing midnight as negative rather than wrapping to the next day.
• Mixing time zones between the two inputs, which silently adds or removes hours.
• Counting both endpoints when the interval is exclusive, the classic off-by-one on date ranges.
• Reading a decimal hours figure as hours and minutes, where 1.5 is ninety minutes and not one hour fifty.