I had an upvote on an old question of mine that happened to be early in the morning Stack Overflow time (UTC), and I noticed that there's no leading zero.
ISO 8601 specifies that all times shall have a leading zero (though durations may omit it).
So, as part of conforming to the ISO 8601 standard, Stack Overflow should show leading zeros for hours before 10 am.
e.g.
[hh] refers to a zero-padded hour between 00 and 24
. I pulled the standard and it actually mandates leading zeroes (section 3.6)