Raspberry Pi: What time is it?  How to add an Adafruit RTC to the Raspberry Pi via I2C. Drew writes -

Unlike most computers, the Pi lacks a real-time clock (or RTC).  This means the Pi doesn’t know what time it is when powered on.  If connected to a network, then the Pi could retrieve the current time from a time server (referred to as NTP).  Another option is to connect an external real-time clock to the Pi via the I2C bus.  I already had a DS1307 Real Time Clock breakout board kit from Adafruit.