The actually watch party starts when the countdown hits zero, by redirecting everyone in the same moment to the video.
"Open the movie directly and ask in the voice chat what time you should jump to" is when you missed the start.
Syncing works via starting everyone at the same time in the beginning of the movie
It's heavyweight and probably better to just sync by somebody shouting a timestamp, but if you need things synced there you go.
Just calculating from the start time at what point in the movie everyone should be. e.g. You are half an hour late, so the others are around minute 30 (Y)
No need to start a new watch party (Y)