Why does my Blackberry Curve 9315 not update the correct time on the screen for?

I had brought this phone from a T-Mobile loyalty program rep to see if i could get a better price than what it was selling for on their website and she was able to give a better price since i have been with them for almost 10 yrs because they do no longer stock Blackberry phones in their retail stores anymore and i have been noticing that the time that is on the phone screen is 10 mintues slow and i can't seem to figure what the problem is and i really need help to figure it out