Clock problem

Not sure where to post this, if it's in the wrong place could the mods move it? Cheers.
For the last two or three days, whenever we switch our pc on, the clock and calender display the wrong time and date. This has a knock-on to my security software and also the program I use to schedule my work. We reset the time and date, but the next time we switch on it's changed again. Any ideas?

 

Windows is getting the time and date from the BIOS when you start the computer. You need to go into the BIOS setup and change it there. Note that if the date and time in the BIOS continues to be wrong each time you start the PC, the CMOS battery may be failing.