Technically it is not important whether the timer is declared in the process_globals of the starter service or the main activity.
The result is exactly the same.
If you want to use the timer from the main module then it should be private and declared in the main module, and initialized when FirstTime is true.
If you want to use it from the starter service then it should be private and declared in the starter service, and initialized in Service_Create.
Both options are good.