Each workstation contains an internal clock powered by a rechargeable battery. After the system is halted and turned off, the internal clock continues to keep time. When the system is powered on and reboots, the system notices that the internal clock has gained time since the workstation was halted.
In most cases, especially if the power has been off for less than a month, the internal clock keeps the correct time, and you do not have to reset the date. Use the date(1) command to check the date and time on your system. If the date or time is wrong, become superuser and use the date(1) command to reset them.