I'm looking for a noun or phrase to indicate a point in time which is either the beginning or end of an event.

Specifically, I'm writing software where I'm logging various events, and I want to use a phrase that's less awkward than "Begin or End Event."

I'm looking for a single word to indicate a point in time which is either the beginning or end of an event.

Edit, for a better example: A bell rings at the start a round of boxing. A bell rings at the end of a round of boxing. Assuming the bell rings at no other times, we can say the bell always rings at the BLANK of a round.

Currently, for BLANK, I'm using the phrase "start or end." It feels clunky. I'd like to replace it with a single word, if possible.