[Remind-Fans] Calculate difference between dates

[Tim Chase]

On 2023-07-18 08:32, Dianne Skoll via Remind-fans wrote:
> > 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.
> 
> REM 20 July MSG Bob's birthday - [$Ty - 2000] years old today.

I have helper functions like these because these are in the vast
majority of my birthday/anniversary reminders:

  FSET born(y) "(" + ($Ty - y) + " year" + plural($Ty - y) + " old)"
  FSET married(y) "(" + ($Ty - y) + ord($Ty - y) + ")"

letting me have things like

  REM Apr 1 MSG Steve [born(1981)]
  REM Apr 1 MSG Jack & Dianne [married(1999)]

-tim