We'd like to know a little more about the context (so why do you need this).
If you generate your diagram daily, you will have a different diagram (since "today" is changing daily :-)
It this correct?
It's not exactly your request, but right now you can have:
!$today = %date("YYYY-MM-dd")
Project starts $today
[foo] lasts 10 days
We should probably add some preprocessor function to handle date/time manipulation.