Do JFK Tape Gaps Add Up To 18.5 Minutes?

From Robert Dallek's An Unfinished Life, his 2003 biography of President Kennedy, Dallek describes the taping system Kennedy ordered the Secret Service to install in July 1992:

Three tapes, [authors Philip] Zelikow and [Ernest] May add, may have been "cut and spliced, for two of these tapes...concerned intelligence issues and may have involved discussion of covert efforts to assassinate Castro." It is also possible that embarrassing passages involving Marilyn Monroe and Judith Campbell Exner were removed. In addition, a small number of tapes may have been destroyed or lost. There are, for example, unopened transcripts at the Kennedy Library for four missing tapes, which may contain embarrassing revelations or national security secrets.