[Remind-Fans] Calculate difference between dates

[aalinovi at riseup.net]

Here is what I would like to do:

If Bob's birthday is July 20, 2000, on July 20, 2023 I would like
remind to print out something along the lines of:

Bob's birthday - 23 years old today.

And on July 20, 2024 Bob's birthday 24 years old today

and so on.

I've tried going thru the man pages and think it has something to do
with substitution filters but not have any programming experience I'm
afraid I really don't understand it.

Any help would be appreciated.

Thank you

Arthur