Question

I need to create clock which should be always synchronized with Android, iOS etc..

Clock should be:
independent. (User or something else could not change time)
synchronized. (time should be the same on Android, iOS etc..)
work offline. (internet connection shouldn’t be required)

I would appreciate for any suggestion.

Now i am looking at gps time.

Answer 1

GPS signal will only be available outdoors.

Instead try using the NTP or SNTP protocol.

(By the way, Android’s internal clock is synchronized using NITZ and NTP/SNTP)
You can search the Android source code for reference implementation.