Reminders app alert time keeps disappearing on iPhone 4s
I use my phone's Reminders app frequently and have started having problems with the "Remind me on a day" option. Today I noticed that whenever I set a time and date for a reminder alert, it disappears after a few moments, leaving just the text I entered but no alert time (ie, a reminder to start dinner at 5:00pm on Tuesday will show up properly on the reminders home screen for a few seconds, and then the alert time will disappear, leaving just "Start Dinner"). Since the alert times keep disappearing, none of my reminders actually work at the moment. I also have weekly reminders that I've used for over a year but starting today they now show up as "(null), weekly" underneath the original reminder text.
I exited the app, restarted my phone and then did a hard reset, but nothing has changed the way the app is behaving. Creating a reminder through Siri just gives me the same problem. I haven't done anything out of the ordinary with my phone so I'm not sure why it's not working properly. Thanks for any help you can give!
iPhone 4s
iOS7.0.6
Apple Reminders App (default iPhone app)
iPhone 4S, iOS 7.0.6, default Apple Reminders app