iphone showing strange dates in all apps
Solution 1:
A quick search into your issue shows you may have your Calendar set as "Buddhist".
You can check this and change that setting by going to the Settings app -> General -> Language & Region -> Scroll down to Region Formats -> Tap Calendar -> Make sure it's set to Gregorian.
Try that and see if that fixes it?
Answer found here: https://discussions.apple.com/thread/4679590?tstart=0