The relative comment times appear to be wrong. I answered a question two days ago as you can see in the screenshot yet the relative time still says it was answered 'yesterday'.
Stack Exchange seems to use a definition of "yesterday", of "between 24 and 48 hours elapsed". See https://stackoverflow.com/questions/11/how-do-i-calculate-relative-time/12#12
So rather than "in previous calendar day", it means "it hasn't been two days since".