You can put any Windows Installer Property as a registry value. We don't have a nice UI for it yet, but the syntax [Valid Property] will get substituted at install time with that property's value.
A complete list of Windows Installer properties can be found at
http://msdn.microsoft.com/library/defau ... erence.asp
Now that being said, I don't know if there is any property with the install date. In that case, you should use the custom action.
Regards,
Cata