Getting System cmos checksum bad error

Phoenix SecureCore(tm) NB 
Copyright 1985-2009 Phoenix Technologies Ltd 
All rights reserved 
Build Time: 05/06/10 12:54:29 
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3896 M System RAM Passed 
256 KB L2 cache per Processor Core 
3072K L3 cache Detected 
System BIOS shadowed 
Video BIOS shadowed 
Fixed Disk 0: WDC WD5000BEVT-75ZAT0 
ATAPI CD-ROM: TSSTcorp DVD+/-RW TS-L633C 
Mouse initialized 
System Configuration Data Updated 
ERROR 
0271: Check date and time settings 
WARNING 
0251: System CMOS checksum bad- Default configuration used 
resume setup 

fter this I press F2 and change the date and time then save it. I can access windows with no problem. After I turn the laptop off again and turn it on after some time this turns up again and I have to change the time again from 12:01 8/20/2009 to the actual time. 

What do you think is the problem?