capturing compile date/time on asp.net page

I have a client that wants to have the time the page was last compiled/built
displayed on a web page. is there any way to have a defined constant or
something in the code behind file to get the build date using some sort of
preprocessor directive.

e.g.
const string dtCompile = $(date)

any suggestion for how to accomplish this without manually updating the
date/time would be appreciated.