I've been through this before but I must be doing something wrong.
My DT Elapsed Timer is incrementing the seconds every 15 seconds.

I'm running a 18F4520 with an External clock of 20 MHZ

I set T1CON = 0 'Main Clock FOSC/4, 1:1 Prescaler .....

In the Elapsed_Int-18 code I verified the constant is 15543... for 100 interrupts per second.

If OSC == 20
TimerConst = 03CB7h
EndIF

What did I miss? Why am I running so slow.