Clouds race over a beach in a thirty-second clip. That speed belongs partly to the presentation: the recording may cover hours. A coastal time-lapse needs two clocks in its caption.
Find the observation interval
Look for the start and end of recording, the spacing between frames and the playback duration. If those details are missing, describe visible changes without assigning them a real-world speed. A short clip alone cannot establish how quickly the shoreline changed.
Check what remains comparable
Camera movement, a changed crop or an interruption in recording can complicate an apparent shift. Follow a fixed landmark when one is available. Do not assume every jump in the image represents a sudden environmental event.
For your own permitted shore-based recording, keep a simple production note beside the finished clip. State any removed intervals and avoid filming identifiable people unnecessarily. A clear caption lets viewers appreciate the changing coast while understanding which rhythm came from the place and which came from the editing process.