Tiny RTC module
This module uses the DS1307 chip, which can be used for keeping time in a very wide variety of projects. For example it can be used to add time stamps to sensor readings, display the time accurately or keep track of time even when there is no power supplied (although it will require a rechargeable CR1225 battery for this). The Tiny RTC supports the I2C protocol, which makes it very easy to interface with many microcontrollers.
VCC: 5V DC
Battery (optional): CR1225
Power consumption: <500nA (in Battery Backup Mode)
RAM: 56 Byte, non-volatile
Memory: 32kb EEPROM
Dimensions: 27mm x 28mm x 8.4mm