How do I calculate the number of days since a record was modified?

How do I calculate the number of days since a record was modified?

We would like to add a field that tells us how many days since the record was modified.  I cannot figure out a formula to do this. I tried this...

Datecomp(${Leads.Today's Date},${Leads.Modified Time})/(24*60)

Keep getting an error about missing quotes. 

All I need to do is subtract two dates, today's date and the last modified date.

in the formula above, Todays Date is a date field we added.  Modified Time is the only field I see in the Formula creation function that seems to apply.

Please help!

THANKS