Github user dyanarose commented on the issue:
I like long myself, but I think that's only because I'm quite used to
working in milliseconds. As the existing static Session Windows take Time as
the gap, I think it made sense to have the extract method also produce a time.
If it returns a Time, we don't have to worry about an implementer getting
confused about what time unit they need to be returning, or always having to
look it up just to check that they're right.