iphone showing strange dates in all apps
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 &amp; 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