There's a local and server playtime. The difference: the local one saves your playtime REGARDLESS of the connection to the Steam server. The server one, you guessed it, needs server connection to normally upload playtime. (local = bigger; server = smaller, if the glitch is performed)
So I have a glitch where my local and server playtime don't sync. This is PERMANENT. And I want to fix it. I'm no Valve employee though. So here is my idea on how to fix it:
If Steam detects a difference between the local and server playtime, a notification would pop up stating that there's an anomaly with the playtime. You can choose to not open it, but the glitch will persist, however, the notification does too. If you click on the notification, you're presented with a choice screen where you can choose which playtime to upload.
-If you were active offline (on a game), you can choose the local playtime as your "real" playtime.
-If you were inactive offline (on a game), you can choose the server playtime as your "real" playtime.