Hi,
Both NAP and SLEEP uses the wathcdog timer of the PIC so the accuracy depends on the accuracy of the watchdog timer oscillator - a figure you should be able to find in the datasheet for the PIC.

SLEEP is programmed in units of seconds so it won't work for you.
NAP can be set to either 18, 36, 72, 144, 288ms and so on so it probably won't work either.

Timers are usually clocked by the main oscillator which is shut down when you put the PIC into sleep.

You probably need to run a timer with an external x-tal or oscillator in order to keep it running when you power down the actual core.

/Henrik.