I'm sure this is insanely obvious... but i can't figure it out.

I have a db that organises applications for an event. Included in the
applicant information is their date of birth. On a form or report i can
calculate and display their current age (=(DateDiff("d",[DOB],Date()))/365)
and also their age at the time (=(DateDiff("d",[DOB],"28/07/07"))/365) of
the event.

As you can see i'm using a hardcoded date for the event date so i have to
change the expression all over the place. How can i set this date in one
location and reference it throughout the application?

cheers,

/mark