Your Mac's conception of what time it is can be very important. Every file you createor save is stamped with this time, and every email you send or receive is marked with this time. As you might expect, setting your Mac's clock is what the Date & Time pane is all about.
Click the Date & Time tab. If your Mac is online, turn on "Set date & time automatically," and be done with it. Your Mac set sits own clock by consulting a highly accurate scientific clock on the Internet. (No need to worry about Daylight Saving Time, either; the time servers take that into account.)
If you have a full-time Internet connection (cable modem or DSL, for example), you may as well leave this checkbox turned on, so your Mac's clock is always correct. If you connect to the Internet by modem, however, turn off the checkbox, so your Mac won't keep trying to dial spontaneously at all hours of the night.
If you're not online and have no prospect of getting there, you can also set the date and time manually. To change the month, day, or year, you can click the digit that needs changing and then either (a) type a new number or (b) click the little arrow buttons. Press the Tab key to highlight the next number. (You can also specify the day of the month by clicking a date on the minicalendar.)
To set the time of day, use the same technique—or, for more geeky fun, you can set the time by dragging the hour, minute, or second hands on the analog clock. Finally, click Save. (If you get carried ...