Clock accuracy Bangle.js 2

How can I prove this theory, is comparing clock drift between the two Bangles over 24 hours good enough for this?

Yes, I think that's probably good enough. One thing you can do is if the Bangle isn't sleeping at all, the SYSTICK timer is going to tick up for every CPU cycle (based on the high speed oscillator) while the RTC counts up with the low speed oscillator - so you could compare how much each one increments...

• read RTC and SYSTICK
  
• for (var i=0;i<10000;i++);
  
• read RTC and SYSTICK
  
• compare
  
  

And hopefully that should give you a comparison between the two oscillators.