Sometimes you get subtitles which are not in sync with the video. You can use ffmpeg to add a fixed delay to all subtitles. Here is how you can do it:

ffmpeg -itsoffset <amount-to-delay-in-seconds> -i input.srt -c copy output.srt

This will create a new file output.srt with all subtitles delayed by the specified amount. You can use this command to delay subtitles by any amount of seconds. Negative and decimal values are also supported. For example:

# This would delay the subtitles by 2.5 seconds
ffmpeg -itsoffset 2.5 -i input.srt -c copy output.srt

Credits: StackOverflow