Calculating length of time with conditions

[markg]

I would like to calculate the length of time a client spends in a program.
We use three columns (entry date, exit date, today's date). If a client is
still enrolled I would like to base the length of time on today's date. If
the client leaves the program, I would like to base the length of time on the
exit date. Preference is to display the length of time in months. Thank you.

 

[Fred Smith]

You want Datedif, as in:
=Datedif(entry date, if(exit date = "",today(),exit date),"m")

Regards,
Fred