The clock time is keep by a battery (not the main one) but a small one on the motherboard when the computer is off.
When the computer is on and online it will automaticaly update the clock time.

Normally that battery last 5 years. 

Try setting the time maualy, by getting the actual update for apple time server.

> sudo ntpdate -u time.apple.com

then you will see something like

> 8 Jun 14:30:11 ntpdate[28267]: adjust time server 17.151.16.14 offset
> 0.000336 sec

You generally don’t need to do this if you use the “Set date and time automatically” feature within the Date & Time settings.

![enter image description here][1]

If that did not work, reset your SMC ([System management Controller][2])


  [1]: https://i.sstatic.net/HYjjn.png
  [2]: http://support.apple.com/kb/ht3964