IIRC, Johnny agreed to do "The Tonight Show" four nights a week (Monday
would have a guest host) in exchange for cutting the show from 90 minutes
to an hour.
By 1980 a number of ABC and CBS affiliates in the Central and Mountain
time zones had added an extra five minutes to their 10 PM newscasts,
and by decade's end the trend was spreading into the Eastern and Pacific
time zones. NBC's affiliates must have demanded the extra five minutes
as well; I think Leno has always started (except for that primetime disaster
two years ago) at 11:35/10:35.
CBS, though, does make a thing about Craig Ferguson actually starting at
12:37 AM (ET/PT); one of my CBS affiliates, WFMY, sometimes has to delay
the network's primetime programming to the wee hours of the morning because
of ACC basketball or Carolina Panthers preseason games, and they'll say that the
delayed network show will start at 2:07 AM or some similar time. (They used to
delay all the CBS soaps during the ACC basketball tournament, where some first-
round games are played in the afternoon; it used to look strange to wake up
early on a Saturday or Sunday morning and see "Young And The Restless," "Bold
And The Beautiful," "As The World Turns," and "Guiding Light" at 5 or 6 AM; I think
they still delay "Y&R," "B&B," and "The Talk," but not "Let's Make A Deal." But I digress.)
I've also noted that, in the past couple of years, WFMY has changed the time of its
Friday-night high-school football highlights show to 11:10, so that Letterman gets on
at 11:35; they used to run the football show at 11:30 and delay Dave to midnight.