How do I set up Reminders in an excel spreadsheet?

[Chilli Mel]

I'm trying to set-up a new spreadsheet in Excel for sales and servicing of
bikes, I want to have something to alert/remind me to contact people for when
their service is due?

[Flintstone]

There are plenty of things you can do, try conditional format with this.

=AND(A1-TODAY()>=0,A1-TODAY()<=30)

Or

Paste this into whatever cell, will returns days remaining from today until the specified date code that is in the formula.

=DATEDIF(NOW(),38779,"d")

Matt