Beginning last night, I have started to receive notifications hours after an event occurs. As an example, I manually watered two zones yesterday at 7:46 PM. Then, after 11 PM, I received notifications that the watering was complete. This occurred again today when my system watered at 8:40 AM and completed two zones at 8:48 AM. However, I received notifications regarding the watering activity around 4 PM.
I do not believe this is an issue with the app on my local device because I carry both an iPhone 6S Plus running iOS 9.3.1 as well as a Nexus 6P running Android 6.0.1. Both phones, which run on entirely different operating systems, produced delayed notifications at the same time.