Elapsed Timer using internal oscillator at low frequency

I need to run a timer for about 24 hours, then reset and run again. Don't need high accuracy ... a few minutes off on each run is OK. Would like to use the internal oscillator. Many PIC parts have int. osc. selections from 125 khz to 8 mhz. The lower frequencies would consume less battery power, which is one of the goals here.

Looking at the DT Elapsed timer program, I see oscillater frequency choices only as low as 4 mhz. How do I modify the program to run at a 500 khz (or lower) oscillator?