Apple will take care of setting the clock on your Mac as long as the Date & Time preferences are correct. Go to the Apple menu, then System Preferences, then Date & Time. In the Date & Time section of the Date & Time preference panel check the box for “Set date and time automatically.” The menu to the right should be the one closest to where you are.
The preferences should have been set this way already but sometimes things happen. With these settings you never have to set the time yourself. Apple will do it for you.