If you remove both the coin battery and the mayfly loses power, even for an instant, the clock will reset to Jan 1, 2000 0:00:00.
The coin battery isn’t necessary if there’s another power supply, but it’s very convenient to power the clock with its own battery so that you don’t have to reset the clock every time you turn the Mayfly off. The clock battery should last for a few years. If you’re running modular sensors and have internet connection, the first thing it attempts to do on boot-up is set the clock and it will continue to attempt to connect to NIST and set the clock on every cycle until the clock passes a “sanity” check (ie, between 2019 and 2025). If there’s no internet connection, though, it will spit out lots of warnings about the incorrect time but should proceed to sample on every 5 minute interval. Once you download the SD card you’ll have to use your installation notes to estimate when you turned it on and adjust everything accordingly.
I don’t remember if there’s a “sanity” check on MMW or not – I thought not. Look for data that’s labeled as 20 years old. (Man that makes me feel old..)