I'm sorry, but that's contrary to the normal dictionary definition of "effective", and it's not what a user expects. "Effective" means doing something, not starting a clock to do something in N+1 days.
Why make it so difficult and confusing for the user to specify the date the first watering occurs? This is a simple user interface oversight that should be trivial to correct, so why stubbornly defend a non-intuitive design error as "by design"?