There seems to be a problem with Reminders and Yahoo Calendar To Dos
Basically, you cannot get a "Remind me on a day" time for any Reminders list synced with your Yahoo Calendars To Do. And it seems to be an issue at the Yahoo end, because Reminders on Mac OS X is doing it too.
If you are still running iOS 7.0.6, what you see is the List entry appear with the time you set, and then once the Activity Indicator spinner in the status bar finishes to show that sync has happened, the datetime gets changed to (null).
If you update to iOS 7.1, it no longer shows (null), it just doesn't show a datetime.
Either way, the "Remind me on a day" slider has gone back to off.
Reminders on Mavericks tries to sync the entry while you are putting the date in, and just unticks "On a date" before you get a chance to enter the date and time!
It only happens on Reminders lists synced to Yahoo, I have other lists synced to iCloud and I have no problems with them.
I know other people have seen this, but they worded their question poorly so it got ignored. Does anyone know a way of raising this with Yahoo?
Plenty of talk about this on the Apple website: