No, it's my fault... not reading the thread properly from the beginning...

What I told you was valid for the timer keeping it's own time...

But, nevertheless if you want the Timer to count 21600 (or however many) pulses from an external source then yes, it's 65536-21600=43936, or $ABA0 that needs to be preset into TMR1H & TMR1L. Note I'm subtracting from 65536 ($0000 or zero) and not from 65535 ($FFFF), because the interrupt is on the roll from 65535 ($FFFF) to 65536 ($0000).

Look at the T1CON Register for your PIC... you will also have to set TMR1CS for EXTERNAL input and provide your 60Hz pulses on the appropriate T1CKI pin for your PIC.