You can adjust the timing offset for an external text based subtitle. You can not change the offset of a subtitle embedded in a video file or image based subtitles.
Supported Plex apps:
- Apple TV app (requires enhanced player, currently in Plex Pass preview)
- Smart TVs (LG 4K/UHD [webOS 2.0+], Samsung 2016+ UHD)
- Xbox One
During playback open the Playback Settings and select Adjust Offsets
- Add or remove from the offset in 50 millisecond increments.
- Click Reset to set it back to a zero offset