Tonight marks David Letterman's 30th Anniversary of hosting a late night talk show. Dave hosted "Late Night" from Feb 1, 1982 to 1993 and will have hosted the "Late Show" from 1993 to Feb 1, 2012).
Dave's 30 years in late night will be celebrated without much fanfare - no special, no "best of" clip show, and no media blitz interviews. The one place it will probably be acknowledged is in tonight's show (probably the Top Ten and the monologue).
Almost all of the news stories about this event mention that, as of tonight, Letterman will be surpassing Johnny Carson who hosted "The Tonight Show" for 30 seasons. It's just another (albeit insignificant) indicator of just how unimportant "facts" and "research" are in today's media. Every news story just parrots every other news story.
In terms of number of days being a late night talk show host, Letterman has already surpassed Johnny Carson. Letterman has reached his exact 30th anniversary (minus 3 months for the transition from "Late Night" to "Late Show", the hiatus for his heart surgery, and the shingles hiatus). And while Carson hosted for 30 seasons, it was 29 years and 7 months in terms of the calendar.
In terms of number of episodes, more of the same. Carson was the host of "The Tonight Show" for 4,531 episodes. Letterman = 1,819 "Late Night" episodes + 3,614 "Late Show" episodes for a total of 5,433 episodes.
It's only in terms of total number of hours of television hosting that it becomes somewhat of a question mark. Johnny Carson's smaller number of episodes is counteracted by the fact that "The Tonight Show" ran 105 minutes per episode from 1962-1966 and 90 minutes per episode from 1967-1980. Letterman's entire run has been in the 60 and 62 minute eras. In order to come up with concrete numbers I'd have to break into the Museum of Television and Radio. As best I can calculate (rough estimates and round numbers though still putting way more effort and thought into it than is called for), it's close but, again, Letterman comes out on top. The difference being the large number of times Johnny had a guest-host (well over 600 episodes) vs. only a handful for Letterman (heart surgery hiatus, shingles hiatus).
By any measure of time, Letterman surpasses Carson tonight in no way whatsoever. It already happened. Thus ends this sports-like examination of late night comedy.
And congratulations on an unprecedented 10,957 days of late-night television hosting.