The Date Meter Became Active is the date you started using the meter, but its best to enter the date of the first bill you have. If the "Date Meter Became Active" does not match your first Bill Date, you will get a warning message.
If you want to enter the real date that the meter came online, and you don't have bills going back that far, you can ignore the warning message. It won't affect your metrics at all.
You can edit the Date Meter Became Active on the Manage Bills page, under "Basic Meter Information"