RTC

The use case demands better than 1 second accuracy over the course of a day. This precludes the use of the simpler real time clocks, such as the one built-in to the Adafruit SD logger (a DS1307), and probably the clock builtin to the microprocessor itself.

The Chronodot, though, is spec'd at 2ppm accuracy for the range of temperatures (0-40°C) we care about.

Access is through a patched version of RTClib, https://github.com/mizraith/RTClib .

So far this has worked as advertised.

With the I2C bus also being used to communicate with the ADC, there is the possibility that it will be difficult to interleave the two. If this is an issue, the second I2C bus could be used, or the square wave output from the RTC could be put onto a digital input of the MCU, driving a counter. That would eliminate any need to communicate with the RTC while sampling was underway.