What kind of errors are you getting? What is your exact code on what template?
Make sure your date fields are in the right case... Add_Date... Mod_Date
The function only returns true or 1 if the first date is smaller than the second.
Also I forgot to add the extra ' at the end of the if statement...
Code:
<%if GT::Date::date_is_smaller($Add_Date, '2006-08-07')%>
yeah!
<%endif%>
This example should work.
Hope this helps,
- Jonathan