While all the Feathers have a LiPo battery backup capability, they don't always have a true "Real Time Clock" that can keep the correct time.
If you have a WiFi or Cellular Feather you may be able to get away with not having a true RTC because you can connect to the network (say, NTP) and query the time. But, even then, there's a chance you'll be off-grid and need to know the time and date.
Thus we have two options for adding an RTC. Both RTC's are I2C, and they share the same address. So you can only have one RTC, but you can match them up with any other Wings. If using the SD card on the Adalogger, just make sure the CS pin is not shared.
Both Wings work with all Feathers. A CR1220 coin cell is required and not included (to make air-shipping abroad easier they are sold separately)
This FeatherWing will make it real easy to add datalogging to any of our existing Feathers. You get both an I2C real time clock (PCF8523) with 32KHz crystal and battery backup, and a microSD socket that connects to the SPI port pins (+ extra pin for CS).
If you don't need to use the SD card socket, leave it empty and the SPI + CS pins can be used for something else.
The PCF8523 used in the Adalogger has an external 32kHz timing crystal that is used to keep time with low current draw. And that's all well and good, but those crystals have slight drift, particularly when the temperature changes (the temperature changes the oscillation frequency very very very slightly but it does add up!)
The DS3231 RTC is in a beefy package because the crystal is inside the chip! And right next to the integrated crystal is a temperature sensor. That sensor compensates for the frequency changes by adding or removing clock ticks so that the timekeeping stays on schedule.
You'll get much higher precision with this RTC, at an additional cost. And there is no space for an SD card holder.