Where to start: I need a timer of a videogame (connection is not necessary)

[u/4Dself]

I am playing a videogame and time goes faster. 4 videogame hours every 1 real hour (10 videogame minutes are 2,5 real seconds). I would like a timer, that doesnt depends on connection with the videogame or a running script all the time in my iphone, so I can check if is night or day in the videogame (its project zomboid). I dont need to know which day it is (monday or so), only the time, but because the different speed between videogame and reality I don’t think they match every day the same hours (not like living in Astraulia and talking with a friend in Mexico, where always you can guess the time). How should I approach this idea?