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For reference, the VTT spec says:

The timestamps can be specified in one of the following two formats:

• mm:ss.ttt
• hh:mm:ss.ttt

Where ttt represents milliseconds and must be between 000 and 999, inclusive.

Also for reference, though there is no official "spec" for SRT, this page summarizes the common conventions:

Timecodes have the following format:

• hours:minutes:seconds,milliseconds

Note that a comma is used instead of a period to separate the seconds and milliseconds.
This is because the SubRib format originated in France.
The first three components (hours, minutes, and seconds) each consist of two zero-padded digits,
and the last component (milliseconds) is three zero-padded digits.

(emphasis added)