Thank you your helping first!
Now I post for details for this case.
P4 520 (28GHz,800FSB,1MB Cache,socket 775)
1GB DDR 400 (Samsung)
ECS 915-m5 (i915GV)
Rehat 8.0,Kernel is orignal 2.4.18
I find out the software time is slower 3 mins every day.
But Hardware Time is correct. (All info find out in webmin)
Every day the server is shut down at 18:00 and startup at 8:30 in morning.
I changed a new otherboard for testing. The case is same. The old motherboard run windows xp home and test the time. No problem found.
Is it Redhat 8.0 problem or something I miss?