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Difference compares two clock labels; Add and Subtract apply a duration to one base time.
Exact preserves the arithmetic; use a minute step only when your schedule or policy requires it.
Nearest uses ordinary half-up rounding at the selected step.
Use 24-hour labels for audit work or 12-hour labels for conversational handoffs.
Standard is concise; detailed is useful for spreadsheet checks.
Optional: for example, Night shift handover.
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The chart renderer is unavailable. The same clock anchors remain available in the ledger.

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A clock label names a position within a day; a duration measures the distance between positions. That difference matters whenever work crosses midnight, a break is removed, or a finish time lands on another day. Without a date, 06:45 does not reveal whether it comes before or after 22:15.

Elapsed duration
The signed or overnight span from a start clock label to an end clock label.
Clock shift
A base time moved forward or backward by a stated duration.
Day offset
The number of midnight boundaries crossed by an addition or subtraction.

Overnight interpretation is a choice, not a property hidden in the two times. Treating an earlier end as tomorrow changes the span by exactly 24 hours. Keeping the entered order instead produces a negative result, which is useful when the order itself needs review.

Break deductions and rounding answer separate questions. First find the raw span and subtract any break. Then, if a schedule or policy calls for it, round the net duration to a stated minute step. Rounding should not be mistaken for measurement precision: a reported 1 hour 15 minutes may have come from an exact span of 1 hour 8 minutes.

Clock-only arithmetic assumes every day contains 86,400 seconds. It suits timetables, shifts, handovers, parking windows, and travel plans that do not cross a daylight-saving change. A dated interval in a known time zone needs calendar-aware arithmetic because a local day can contain 23 or 25 hours.

How to Use This Tool:

Choose the operation before entering times so the earlier-end and rounding choices apply to the right kind of calculation.

  1. Choose Difference for an elapsed span, or choose Add or Subtract to shift one base time.
  2. Enter clock labels in 24-hour, compact, or 12-hour form. Duration fields also accept values such as 01:15, 75m, or 1.25h.
  3. For a difference whose end is earlier than its start, select Carry to next day only when the event truly crosses midnight. Add a break deduction when it belongs outside the measured time.
  4. Leave Rounding step at Exact unless a real policy requires a 1, 5, 10, 15, 30, or 60 minute interval. Then choose nearest, up in magnitude, or down in magnitude.
  5. Compare the raw, exact, and reported values in Time ledger. For clock shifts, also check the day offset before using the displayed clock time.

Interpreting Results:

For a difference, the reported duration is the net span after the break and optional rounding. A negative value means the entered end precedes the start under the signed interpretation. For addition or subtraction, read the clock result together with Day; 00:35 with +1 day is not the same moment as 00:35 on the starting day.

The Rounding check states whether the displayed result moved away from exact arithmetic. Keep the exact value when the rounded answer must be audited or reconciled with a timesheet.

Technical Details:

Every accepted clock label is converted to integer seconds from the start of a fixed 24-hour day. Durations are also reduced to integer seconds, with fractional unit entries rounded to the nearest second before clock arithmetic begins.

Formula Core

The elapsed calculation subtracts the start from the end, optionally adds one day, and then removes the break:

Dnet=Tend−Tstart+C×86400−B

Here, Tstart and Tend are clock seconds, C is 1 only for an earlier end carried to the next day, and B is the break in seconds. The add and subtract modes use a signed duration instead:

Texact=Tbase±D

Optional rounding works on the magnitude, then restores the sign. Let X be the net duration in Difference mode or the exact base-plus-duration clock total in Add and Subtract modes. With a selected step of s seconds, nearest uses ordinary half-up rounding; up uses the next larger magnitude; down uses the next smaller magnitude.

Xreported=sign(X)×R(|X|s)×s

For example, 22:15 to 06:45 carried overnight is 30,600 seconds. Subtracting a 1,800-second break leaves 28,800 seconds, or exactly 8 hours.

QuantityAccepted ruleBoundary
Clock timeHours, minutes, optional seconds, and optional AM or PM24:00 is valid; later values and 24:00:01 are not
DurationClock-style or unit-labelled positive value0 through 365 days, inclusive
BreakBlank means zeroMay exceed the raw span, producing a negative net duration

Accuracy Notes:

The model has no date, time zone, or daylight-saving rule. It also does not model leap seconds. Use a dated, zone-aware system for payroll, travel, logging, or legal records when a clock change or jurisdictional rule can alter the true elapsed time.

References: