Drop-frame timecode is not a subtitle timestamp

The same position can be described as a frame index, a timecode label or a number of elapsed seconds. These descriptions are related, but they are not interchangeable strings. A colon in an editing label does not mean the last field is milliseconds.

Apple's timecode technical note explains how drop-frame numbering compensates for fractional-rate timing by skipping labels. It does not remove pictures. Use that distinction before interpreting a frame calculation as a media change. Apple timecode reference.

Identify the unit before calculating

A label such as 00:01:00:00 at 25 fps NDF describes a frame position. An SRT timestamp such as 00:01:00,000 describes sixty elapsed seconds. At a different frame rate, a similar-looking NDF label can represent a different elapsed duration.

Fractional rates are represented by rational values: for example, the common 29.97 label corresponds to 30000/1001 frames per second. Rounding that to an integer for long calculations introduces drift. Keep the chosen rate attached to the result.

A worked example

At 25 fps NDF, one minute contains 1,500 frames and thirty seconds contains 750 frames. Adding those positions as durations gives 2,250 frames, displayed as 00:01:30:00. This arithmetic does not involve an uploaded video.

At 29.97 DF, the label after 00:00:59;29 is 00:01:00;02. The skipped labels do not represent missing frames. A calculator should reject a nonexistent label rather than silently treating it as valid input. Every tenth minute follows a different label boundary.

Separate conversion from retiming

The timecode calculator converts a position by preserving elapsed time and rounding to the nearest target frame. It reports the rounding difference. It does not make footage faster, add frames, change a recording or guarantee a conform operation in an editor.

For clip duration, decide whether the out point is included. An end-exclusive interval from frame zero to frame twenty-five contains twenty-five frames, not twenty-six. State the convention when exchanging numbers with another editor.

Review before handoff

Record the input rate, DF or NDF convention, operation and output rate beside the result. Check minute, ten-minute and hour boundaries when fractional rates matter. A calculator that looks correct on a five-second example can still fail at those boundaries.

Read timecode for terminology, and use the subtitle editor when the task is millisecond caption timing. Moving between these tools should be a deliberate change of units, not a guess based on punctuation.

References checked November 4, 2026. Product descriptions are from public official material, not paid-output testing. Examples and cover artwork are original Topicut editorial material.