Specifically for sent, expires, and onset, I suggest a Note to the effect of

"If the location cited in the area block falls within a single timezone, effective SHOULD specify time in that zone, including allowance for Daylight Savings when applicable."

The rationale is that times in the correct time zone are easier for users to understand and take action on, but frequently alerts use UTC. Alert disseminators aren't always in the best position to know the time zone of the user to which they are disseminating the alert.