How to change start time using ffmpeg?
FFmpeg is simply telling you the starting timestamp as parsed from the input. MPEG-TS is a transport stream format, used to convey television broadcasts..etc, having no global header, and can be sliced at ~arbitrary points. There are timestamp packets (PCR) at regular intervals, and ffmpeg is showing you the timestamp of the earliest packet.

If you want your TS to have a start time of 0, you'll have to remux it.

ffmpeg -i myRecordedFile.ts -map 0 -c copy -muxpreload 0 -muxdelay 0 newFile.ts


..but this is unnecessary.