a day when one works only in the morning or only in the afternoon.

What's a half a day?

countable noun. A half-day is a day when you work only in the morning or in the afternoon, but not all day. "If I could have just what I wanted," Sharon mused, "I'd work half days."

What time is a half day?

Hence, half day work is composed of 4.25 hours - the first half is from 8:00 AM - 12:15 PM and the second half is from 12:45 PM - 5:00 PM.