I recently upgraded my 2011 13" Air with Parallels 7 and am not having this issue, but if I had to guess I'd say that Parallels isn't causing the problem.
I'd recommend going to System Preferences and entering the Date & Time section, then make sure the checkbox for "set date and time automatically" is checked and that it is set to the correct region.
In order to make sure that Parallels isn't the problem you can go to the Configure menu for Parallels, then to the Options section and the Advanced tab. Under the Time dropdown, make sure it is set to sync to Mac OS X.
If these steps don't work, let me know, maybe I can figure something else out.