but still for timing applications you need an external Xtal, INTRC its not enough after some days running
Sounds like a test is in order. Are you saying that after a few days the clock is no longer at said speed? by how much? I can see a test such as this:

Code:
main:
 toggle port pin
 goto main
this will give some freq out. so if we let it run a few days, we can see the new freq out.