@gmac Good question, currently it is for the whole device, but I understand your use case.
You essentially want to manually skip the next time a scheduled schedule runs.
Few options. You can disable the watering time in the app, but will have to remember to enable it.
I just submitted more actions to IFTTT that will allow you to skip a watering time. These new actions are skip watering time, pause, resume, and skip zone. They should be approved this week.
So, you could create a recipe that when you send an SMS or email, it will skip that watering time. It will run the next time so you don't have to worry about re-enabling.
I know it's not perfect but that should do the job. I don't see us (in the near term) adding the functionality to rain delay watering times. The best in app choice would be to disable the watering time and re-enable when you see fit.
Hope that helps!