Hi @Philip_Collins welcome to Zwift forums.
Shuji at Zwift HQ here.
Just to confirm I’m understanding correctly - when you end a ride - your activities save with the correct date and time in your Zwift activity feed, is that right? This means that your FIT file uploaded to the Zwift server correctly before our servers pass it along raw to the third party servers. Zwift does not alter the FIT file in that process.
Question 1: it’s possible something might be messed up with the Zwift game app on your end, and if that’s the case - a complete deletion of everything Zwift and re-install might help. Maybe. This is a deeper clean than the usual “Uninstall” process. Here’s the how-to instructions.
Q2: are you running any game hacks (altering XML files, or using an overlap app)? If yes - disable those and see if that might be getting in the way.