Computer time and date getting reset or losing time
Computer CMOS battery failing or bad
The date and time will commonly get reset to the BIOS manufacturer date or epoch, or a default date such as a 1970's, 1980's, or 1990's date. When this occurs, this is a good indication that the CMOS battery is failing or is already bad.
Before replacing the battery, set the date and time to the correct values in CMOS setup and save and exit the setup.
If, after saving the values, the date and time are reset when turning your computer off and on, set the values again but this time leave your computer on for 2-3 days without turning it off. In some cases this can help enable the CMOS battery to retain its settings for longer.
If this doesn't resolve your issue you will need to replace your CMOS battery.
It is possible that older computers may not have a BIOS that is compatible with any year 2000 dates. If your computer was manufactured before 1995 it is recommended that you contact the manufacturer of the computer or motherboard manufacturer to determine if the computer motherboard BIOS is Y2K compatible.
Issue with APM
Third-party utility or program
Third-party utilities or programs or screen savers can cause the time to stop or decrease significantly. If you are running Windows close all screen savers and End Task all TSRs and disable all screen savers to ensure your lost time is not being caused by this issue.
If this does resolve your issue, reboot the computer and determine what TSR or screen saver was causing this issue. Once the culprit has been located, see if the program has any available updates to resolve your issue.
Windows 95, Windows 98, or Windows ME user
When changing the year in Windows 9x or Windows ME, the time will stop until the Apply button is pressed.
When changing the month or date in Windows 9x or Windows ME, the time may decrease by 5-10 seconds.