from wikipedia...
Real-time clock
The Raspberry Pi does not come with a real-time clock, which means it cannot keep track of the time of day while it is not powered on.
As alternatives, a program running on the Pi can get the time from a network time server or user input at boot time.
A real-time clock (such as the DS1307) with battery backup can be added (often via the I²C interface).
Michael.
