The PCF8583 is a real time clock IC with a small bit of additional memory to boot. Looking to buy or find the datasheet. Look here.
I2C-bus interface operating supply voltage: 2.5 V to 6 V
Clock operating supply voltage 1.0 V to 6.0 V at 0 °C to +70 °C
240 × 8-bit low-voltage RAM
Data retention voltage: 1.0 V to 6.0 V
Operating current (at fSCL = 0 Hz): max 50 μA
Clock function with four year calendar
Universal timer with alarm and overflow indication
24 hour or 12 hour format
32.768 kHz or 50 Hz time base
Serial input and output bus (I2C-bus)
Automatic word address incrementing
Programmable alarm, timer, and interrupt function
Slave addresses: 0xA0h or 0xA2 depending on jumper setting.
Here you can see the schematic and next to that that a picture. The IC that goes next to the PCF8583 is a 24LC16 memory chip about which there is another article.
The output from the library can be seen in the screenshot. It shows how you can update the time by sending YYMMDDhhmmss; to the arduino.
The semicolon is needed to indicate end of the message. You do this in the serial window of your pc. Simply type the correct string and press send. The library has been updated to work with the arduino 1.0 environment.
It can be downloaded here including the example to drive it. The library has been written by Erik DeBill
PCF8583 program output.
Links to PCF8583
Here are some more links to other sites talking about this chip and the arduino.
This guy seems to pretty much have done it all.