For everyone having the same problem, you should use a singleton instead of making the variables “static”.
I guess having static network variables doesn’t make much sense ^^
You are making your network variable depend on a single machine’s delta time! That spells trouble! This may result in fluctuating time depending on who is updating the timer and whatever hiccups that machine may currently be experiencing.
You should be using the already synchronized time which is exposed in the NetworkManager as LocalTime and ServerTime. It is measured in ticks, as in “concrete steps” since the concept of “delta time” should not be applied when it comes to updating networked game state.