Subtract positions, not printed digits
Duration frames = end frame position − start frame position. The out point is exclusive: from frame 0 to frame 24 contains 24 frames, not 25. For an overnight crossing, add one 24-hour timecode cycle only if the end is earlier than the start.
Worked example
At 24 fps, 01:00:00:00 to 01:01:30:00 is 00:01:30:00 = 2,160 frames = 90 seconds. At 29.97 DF, 23:59:59;29 to 00:00:00;00 is one frame when “Cross midnight” is selected.
Assumptions and limits
Both points must share the selected frame rate and DF/NDF convention. Without midnight crossing, an earlier end produces a negative duration. Midnight mode accepts only nonnegative clock labels below 24 hours; identical endpoints mean zero, not a full day.
For supported rates and input formats, see the frame-rate reference on the calculator home page.