This problem happens all the time and it drives me crazy. I will add, commit and push a file up to github and it will display the wrong time as to when it was committed. Sometimes it will show that the commit happened a few days prior to when it actually was just added, committed and pushed (which rules out a simple timezone discrepancy). Even when I initialize a brand new repository and commit a file immediately afterward, it will say that it happened 2 days ago.
If anyone knows of an article or previously answered question that deals with this issue, I would greatly appreciate you pointing it out to me.