SRT timecode checker and fixer

The SubRip (.srt) timecode format

Every SubRip timecode line is the same shape, and it is the part players are strict about:

Block structure

A .srt file is one block repeated until the transcript runs out. Each block is four things in a fixed order:

1. A sequence number on its own line, counting up from 1.
2. The timecode line: start time, a space, the arrow -->, a space, end time.
3. One or more lines of subtitle text.
4. A single blank line, which is what actually ends the block.

A real .srt file

1
00:00:00,498 --> 00:00:02,827
Here is the first subtitle line.

2
00:00:02,910 --> 00:00:06,331
A block can carry two lines,
and the second one sits right under it.

3
00:00:06,415 --> 00:00:09,999
The file ends with a blank line after the last block.

Timecode fields

HH

Hours, always two digits. 00 for anything under an hour.

MM

Minutes, always two digits, 00 to 59.

SS

Seconds, always two digits, 00 to 59.

mmm

Milliseconds, always three digits, 000 to 999.

,

The separator before the milliseconds is a comma. A period is WebVTT, and it is the single most common reason a player rejects an .srt file.

-->

Two hyphens and a greater-than sign, with exactly one space on each side. Not an en dash, not "->", not "--->".

SubRip against WebVTT

WebVTT (.vtt) is the same block structure with four differences: the file opens with a WEBVTT line, milliseconds are separated by a period, the hours field can be dropped, and cue identifiers do not have to be numbers. The converter above moves a file between the two without touching the timing.