Is it losing seconds or has it always been behind?
If it has always been behind, go to "Adjust date/time", Internet Time, Change settings, Synchronize with an Internet time server, Server: time.windows.com or time.nist.gov.
Press Update now. Press OK.
If you've already tried that, then changing the battery, as suggested would be the next thing to do.
System Manufacturer/Model Number Self-built OS Windows 7 Home Premium 64 bit CPU Intel Core i7-3770 Motherboard MSI Z77A-G45 Memory G.SKILL 8GB (2 x 4GB) DDR3 F3-10666CL9D-8GBNT Graphics Card Sapphire ATI Radeon HD 6670 Sound Card On-Board Realtek Monitor(s) Displays Samsung SyncMaster P2370HD, Dell 1703FPT Screen Resolution 1920 x 1080, 1280 x 1024
Keyboard Logitech K270 Mouse Logitech M705 PSU SeaSonic M12II SS-500GM Case Lian Li PC-9F Cooling Zalman CNPS9900ALED Hard Drives Samsung HD103SJ Internet Speed 16 Mbps Other Info Bose Companion 2 Multimedia Speakers