Most likely the CMOS battery inside the laptop has died and needs replacing.
For such a small component there's a ridiculous amount of dis-assembly involved to get at it.
There's a step-by-step guide here for the ThinkPad T41 but your model may differ slightly: http://www.ifixit.com/Guide/Repairing+IBM+ThinkPad+T41+...
Note which way up it is (positive side up, or negative side up), remove it, then take it to a battery or PC components shop and buy one that's the same size and voltage.
Put the new battery in the same way up as the old one was.
Hello,date & time not set properly & get the pc shutdown ,then time & date is back date
When you restart and get the date and time back, do you need to adjust the date and time to current values? If you have to do this then the RTC (Real Time Clock) aka CMOS battery is bad; it is a CR2032 lithium cell. I tried to replace this in a Compac laptop and it was not worth the trouble.
If on restart the date and time is correct, then right click on the 'time' logo on the taskbar (lower right of screen) and make sure that the time is synchronized with Internet Time or some other appropriate standard.