Why are my iPhone photos showing incorrect time stamps?

Abbas Sarfaraz Khan wrote: … Surprisingly all videos show the same original and adjusted time.

There is no method within Photos of changing the capture time on a bunch of picture to the same time. Time adjustments done with Photos are relative-- a group of selected pictures' times are changed by the same amount,

rather than to the same amount.

So, my thoughts are that this happened outside of Photos, perhaps by some 3rd Party app that you have that does things you didn't realize, or that they just didn't tell you about.

Abbas Sarfaraz Khan wrote: … I find it difficult to believe that apple would allow other apps to take precedence over their own native software

Many apps have access to the Photos Library-- Messages and Safari for instance. The point is that this is happening to you, but not to me or others, so you should look for differences.

Have you tried a forced restart? 

Force restart iPhone - Apple Support

This is different from a simple Power Off. I had to continue holding the button down past the "slide to power off" message, and then I released the button only after  the Apple Logo appeared indicating a re-start.