Hi Flotul. I've used a code similar to this in the past and found it to be accurate within 2 minutes per hour using the internal oscillator set to 4 MHZ. Also, I operated it at 3V (using an LF series PIC), current consumption was only 1/2 mA and runs a long time on 2AA alkaline batteries.


x var word
let x = 0

start:
pause 1000
let x = x + 1
if x >= 900 then (15 minutes passed, do your measurement)
goto start