I'd stay away from local time and all its issues.
Trying to calculate deltas and durations with a broken
down local time like HH:MM:SS MM/DD/YY is an absolute mess.
Epoch based timing makes things so much simpler so it is often best
to stick with something like Unix time.
I'd recommending using
using Michael's Arduino time & timealarms library:http://www.arduino.cc/playground/Code/Time
which does all that for you.
With that you can simply set an alarm to call a function.
In your case you could have it call a function to start the watering and another to stop the watering.
The timealarms library has some useful features that allow you do things based times
every day or only on particular days of the week.