Your comment tells me that you define a day as a half-open interval, and you choose to make midnight part of the "following day"

No, the parallel of what i said for days would be: if one defines a day to be 24 hours that starts at midnight, then midnight would be part of the following day.

The people to which you refer define "day" differently, a period of time that starts at midnight and ends at the next midnight.

I was addressing the OP's question

I know you were trying to answer why it's "open by default" as the OP claimed in his question. The problem is that your answer accepted the premise of a default when there isn't one.

I agree about you about the ease of defining span as [...), but it's not the reason for some non-existing default.