Updated June 15th, 2019.
When the power comes back on after any length of outage, and you see these blinking “12:00” numbers on so many devices, do you think “Gee, they really ought to put a battery in these things so the time wouldn’t reset”? I know I do.
And… when Daylight Saving Time comes around, and you have to change all of your clocks, do you think “Gee, this out to be automatic by now, they’ve been doing it for a zillion years”? Of course you do.
Well, where your Macs are concerned, Apple will take care of setting the clock as long as the Date & Time preferences are correct. Go to the Apple menu, then System Preferences, then Date & Time. If you have a fairly modern Mac you’ll see a lock in the corner. If so, click it, and enter your admin password (the one you use to unlock your Mac, and install software). The lock is there to keep unauthorized people from messing around with your Mac’s settings.
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. This way, the date and time are obtained from the internet.
While you’re in there, check the Time Zone settings. It should look something like this (whether automatic or not, it should show the proper time zone):
(If you travel with your Mac it’s nice to alway have it showing the local time. If you prefer to always show your “home” time zone, uncheck the box that sets it automatically, and choose your time zone manually.)
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. If your Mac’s clock seems wrong, you probably don’t have it set to “Set date and time automatically.” Now you know how to fix it.