DIYmall DS3231 I2C Pi RTC Module with CR1220 coin cell battery Holder for Raspberry Pi (Pack of 2pcs) FZ3888X2
This is the best battery-backed real time clock (RTC) you can get that
allows your Raspberry Pi project to keep track of time if the power is
lost. Perfect for data-logging, clock-building, NTP servers,
time-stamping, timers and alarms, etc. Equipped with a genuine DS3231
RTC, it works great with the Raspberry Pi and has native kernel support.
Works perfectly with any Raspberry Pi (or similar single board linux computer) including the Pi 1, Pi 2, Pi 3, Pi Zero, A+, B+ etc.
Note: This product does not come with a CR1220 coin cell battery.
Product Dimensions:2.2cm x2.41cm
Will keep time for 5 years or more
2 X DS3231 I2C Pi RTC