Even on the most trouble-prone of computers, your system clock is one thing you'll seldom have to worry about. It simply keeps the time, year after year, with little need for any input from you. You will occasionally need to correct the time if you've moved between time zones or purchased a machine with the operating system already installed, but the process isn't difficult. If you're working in Windows 7 or the Windows 8/8.1 Desktop, it's as simple as clicking the taskbar clock itself.
A Quick Tweak
If you click the clock with your left mouse button, as you normally would, you'll see a popup clock/calendar with a link at the bottom reading "Change date and time settings..." Alternatively, if you click with the right mouse button, you'll see a popup menu with "Adjust date/time" as one of its options. Either choice takes you to the same Date and Time dialog box. Click "Change date and time..." to make your changes. If you're working from the Windows 8/8.1 Modern interface with its tiled screen, a few more steps are required. Swipe in from the right, or point your mouse to the screen's top right corner, to bring up the Charms bar. Choose "Control Panel," then "Change date, time, or number formats" from the Clock, Language and Region group of settings.
A Power Outage
If you find your older computer won't keep the correct time, it's usually because your computer's memory backup battery is failing. This is usually a coin-shaped lithium battery, located on the motherboard of your computer. They're rated for five to seven years' life, but can occasionally malfunction sooner. Laptop manufacturers typically place it under a removable cover to allow for easy replacement. Consult your user's manual, or the manufacturer's website, for instructions for your specific computer.