This issue is being faced since quite a while on my iPhone 5c since iOS 9 upgrade. All the dates wherever applicable are getting displayed are something not-in-human understandable format. I tried complete restore, waited for updates till 9.3, also have asked for apple authorized service center. None helped. They said, its not a product fault.
Attaching screenshots. these screenshots are old, but the issue still presists.
Best Answer
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