Hi,
Some rules about timer-for-time I use in my projects when I need with real clock ticks without unexpected divergence:

- set the maximal priority level for a timer interrupt; another ones should have lower level.

- never stop a timer! If you need to avoid timer ISR by a some reason then there is another way: masking interrupt. For example, somewhere in main program do not use

CLR TR0
[some framefork]
SETB TR0

but instead

CLR ET0
[some framefork]
SETB ET0

Moreover, if you know that a program has interrupt(s) with long-time ISR(s) execution then instead CLR/SETB ET0 use CLR/SETB EA.

- use auto-reload feature of timers 0/1/2.
- use a good OSC.
You may add/delete some rules by your taste.
Good days!