Google Sheets mortgage functions

Ensure that you use opposite signs for payment and principal with functions like NPER. I blithely expect money functions either to assume the correct sign, or to produce the correct result with the wrong sign.

So for example with a 5% mortgage, principal £110,000, payment £643 per month:

=NPER(5%/12, 643, -110000)

and

=NPER(5%/12, -643, 110000)

both give the correct answer (~ 300 months/25 years), whereas

=NPER(5%/12, 643, 110000)

gives the answer -129, which one might easily mistake for 10-11 years if in the habit of mentally knocking off an unexpected minus sign.