The High Speed Oscillator is shut down in SLEEP Mode, therefore the WDT relies on the 32 kHz LFINTOSC. This can be internal or external. You didn't mention which PIC you're using, but the WDT will have scalers (Pre- or Post-) to divide the 32 kHz for longer durations. The more clock divisions, the less precision you can achieve. You may need to create a Variable that can be incremented each time the WDT wakes the PIC from SLEEP. When the Variable reaches a predetermined value (equating to your 12 hours), you then go to your MAIN Function and do some work. Some PICs will allow you to achieve 12 hours in a single session. The Data Sheet will give you the details for your chosen PIC.