Does this account for daylight-saving transitions?

No. This is pure clock arithmetic on a single calendar day. If your shift crosses a DST switch (spring-forward / fall-back) the wall-clock duration differs from the elapsed-real-time duration by one hour. Use a date-and-time calculator that takes timezones into account for that case.

Can I enter the time without seconds?

Yes — HH:MM is treated as HH:MM:00. A bare integer is treated as a whole-hour time on a 24-hour clock (so "9" means 09:00:00).

Why is "Decimal hours" useful?

Most payroll systems and engineering spreadsheets store time as a decimal-hour float (e.g. 8.5 for an 8h 30m shift). Decimal hours is what you paste into them.

Is the result accurate to the second?

Yes. Internally the calculator works in integer seconds, so there is no floating-point drift in the HH:MM:SS headline.

Does this account for daylight-saving transitions?

No. This is pure clock arithmetic on a single calendar day. If your shift crosses a DST switch (spring-forward / fall-back) the wall-clock duration differs from the elapsed-real-time duration by one hour. Use a date-and-time calculator that takes timezones into account for that case.

Can I enter the time without seconds?

Yes — HH:MM is treated as HH:MM:00. A bare integer is treated as a whole-hour time on a 24-hour clock (so "9" means 09:00:00).

Why is "Decimal hours" useful?

Most payroll systems and engineering spreadsheets store time as a decimal-hour float (e.g. 8.5 for an 8h 30m shift). Decimal hours is what you paste into them.

Is the result accurate to the second?

Yes. Internally the calculator works in integer seconds, so there is no floating-point drift in the HH:MM:SS headline.

Time Difference Calculator (HH:MM:SS)

Subtract one time from another and read the result as HH:MM:SS plus total seconds, minutes, and decimal hours. Handles cross-midnight spans automatically.

Result

Duration

08:30:00

Total seconds30,600
Total minutes510.0000
Decimal hours8.5000
Plain English8h 30m 0s

Source: ISO 8601-1:2019 §5.5 — Time-of-day durations.

Note — Same-day clock arithmetic only — no DST/timezone correction. For schedules that cross a DST boundary use a date-aware calculator.

Step-by-step

Parse start → 32400 seconds-of-day; end → 63000 seconds-of-day.
Compute end − start = 30600 seconds.
Convert seconds back: hh = floor(diff/3600), mm = floor((diff%3600)/60), ss = diff%60.

How to use this calculator

Enter the start and end time in either 24-hour or 12-hour format.
Pick the cross-midnight behaviour you want — Cross-midnight is the right choice for overnight shifts and travel times.
Read the HH:MM:SS headline or any of the equivalent total-time rows underneath.

About this calculator

Subtract one wall-clock time from another and you get a duration — but the moment the end time wraps past midnight, naïve subtraction gives you a negative number that does not mean anything physical. This calculator handles the wrap explicitly: pick the behaviour you want (span crosses midnight, auto-swap to always positive, or strictly report the negative) and read the result both as a HH:MM:SS string and as cumulative seconds, minutes, and decimal hours. Inputs accept 24-hour (09:30, 17:15:00) or 12-hour with am/pm (9:30 am, 5:15 pm). Useful for time-card math, race-split arithmetic, and night-shift duration.

How it works — the formula

diff = end_seconds − start_seconds
if diff < 0 and mode = cross-midnight: diff += 86400
HH:MM:SS = (⌊diff/3600⌋, ⌊(diff%3600)/60⌋, diff%60)

Wall-clock time is a 24-hour cyclic group; subtracting modulo 86400 is the standard handling for spans that cross midnight. Below midnight, integer-second arithmetic avoids the FP precision quirks that show up in spreadsheet time math.

Sources: ISO 8601-1:2019 §5.3 — Time of day · POSIX.1-2024 — Time formats and intervals · NIST Time and Frequency Division — Time scales and conversions

Worked examples

Example 1

Standard day shift

Inputs:: 09:00 → 17:30
Output:: 08:30:00 (8.5 h)

Example 2

Overnight shift

Inputs:: 22:00 → 06:00 (cross-midnight)
Output:: 08:00:00 (8 h)

Example 3

12-hour input

Inputs:: 9:30 am → 1:15 pm
Output:: 03:45:00

Limitations

No DST / timezone correction — pure clock arithmetic on a single calendar day.
Same-day spans only (or a single midnight crossing). For multi-day durations use a date-time calculator.
Bare integers are interpreted as hours; for minute-only inputs include the colon (e.g. "0:45").

Integer-second arithmetic — results are exact for any valid input.

Frequently asked

If your end time is numerically smaller than your start time (e.g. 22:00 → 06:00) the calculator interprets it as "the next morning" and adds 24 hours to the difference. So 22:00 → 06:00 returns 8h 0m 0s, not −16h.