How Time Flys is a comedy album written by David Ossman and featuring the voice talents of all four members of The Firesign Theatre plus several other contributors.

David Ossman first created the Mark Time character as a parody of Flash Gordon for a November 1970 episode of The Firesign Theatre's radio show Dear Friends.

In 1972, inspired by a news story about a hypothesized tenth Planet X, he wrote a story line with the character being sent on a voyage to Planet X, for use in the radio broadcast and movie Martian Space Party, recorded on the album Not Insane or Anything You Want To.

Side one: NIGHTSIDE—DECEMBER 31, 1999 (24:10) Side two: DAYSIDE—DECEMBER 31, 1999 (16:45) The Stereo LP includes a stiff paper insert that can be broken apart and assembled to form a "3-D Diorama" of the album cover art, which shows Mark descending from the Zeppelin over Panoramaland 2000.

The LP insert states that the album was written and produced from March 11, 1973 to June 7, 1973 "in Wally Heider Studios 3 & 4 and on location".