The PCF8583  is a real time clock IC with a small bit of additional memory to boot. Looking to buy or find the datasheetLook 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.


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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.


PCF8583 software

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.