I have a zone that has been running once every 5 days. Yesterday, I noticed that the zone was getting overwatered, so I decided to change it to once every 6 days. To my surprise, it ran again today!
The reason is that the schedule had a Start Date that was 36 days ago, so yesterday was a multiple of 5 and today was a multiple of 6.
I wish the app had noticed that I was changing the frequency and asked me whether I wanted to reset the start date to today.