A RTC (Real Time Clock) module allows your microcontroller project to access the date and time.
This lets you write code, for example, to turn on the lights at 8am and turn them off at 5pm, every day except the weekends.
It has space for a CR2032 battery on the back, which means it will keep track of time even when the power is lost.
Being plugged into 5V will also trickle charge the lithium cell battery.
We have two modules, the DS1307 (left) and DS3231 (right). They are both RTCs, by different manufacturers, with different form factors. The DS1307 has additional space for a DS18B20 temperature sensor.
NOTE: We have removed DS3231 from the options below, because of warnings about the dangerous lithium battery charging circuit used on them. You can still request them. But use at your own risk.
To wire correctly, connect 5V to VCC, GND to GND, and if your Arduino has SDA and SCL, connect SDA and SCL to those, otherwise Analog 4 to SDA and Analog 5 to SCL. That's it.