Question: What period of time was the serial killer that inspired the film Dirty Harry active?

Context paragraphs:
Dirty Harry novels: "Dirty Harry" novels include film novelizations and original tie-ins based on the "Dirty Harry" film franchise.  Like the films, the novels portray Inspector "Dirty Harry" Callahan as he ruthlessly fights criminals.

Dead Right (film): Dead Right is an early short film by Edgar Wright, later famous for the TV programme "Spaced" and the film "Shaun of the Dead".  It was filmed in 1992 and 1993 in his hometown of Wells, England when Wright was only 18.  He wrote, edited, produced and directed the film as well as shooting and recording the sound.  It is a Zucker Brothers-style comedy that parodies the action thriller genre, most notably the "Dirty Harry" series ("Dead Right" was the working title for the original "Dirty Harry").  The film is shot on SVHS and contains an impressive cast of 70 actors (mostly amateur), mainly made up of Wright's school friends and colleagues.  Clips from the film were first broadcast on "Take Over TV" – the Channel 4 series consisting entirely of video clips sent in by viewers – that also launched the careers of comedy duo Adam and Joe.

Sudden Impact: Sudden Impact is a 1983 American action thriller and the fourth film in the "Dirty Harry" series, directed by Clint Eastwood (making it the only "Dirty Harry" film to be directed by Eastwood himself), and starring Eastwood and Sondra Locke.  The film tells the story of a gang rape victim (Locke) who decides to seek revenge on the rapists ten years after the attack by killing them one by one.  Detective Callahan (Eastwood) famous for his unconventional and often brutal crime-fighting tactics is tasked with tracking down the serial killer.  As the detective investigates the killings, he becomes romantically entangled with the woman, and he is drawn deeper into the mystery surrounding the murders.

Dirty Harry (2007 video game): Dirty Harry is a canceled video game that was in development by The Collective, Inc. intended to be published by Warner Bros.  Interactive.  The game was to continue the story of the 1971 film of the same name starring Clint Eastwood as Harry Callahan, the protagonist.  Clint Eastwood was intended to reprise his role, lending his voice and likeness as well as consulting and creative input.  The game was to follow the same storyline of the film, with the San Francisco detective tracking down a serial killer named Scorpio.  Versions were planned for both the PlayStation 3 and Xbox 360, with a release in 2007.

Dirty Harry (1990 video game): Dirty Harry is a 1990 video game based on the "Dirty Harry" film franchise starring Clint Eastwood.  Though the game is sometimes incorrectly referred to as "Dirty Harry: The War Against Drugs", nowhere on the box or in the game does it state that subtitle.

Dirty Harry: Dirty Harry is a 1971 American action crime thriller film produced and directed by Don Siegel, the first in the "Dirty Harry" series.  Clint Eastwood plays the title role, in his first outing as San Francisco Police Department (SFPD) Inspector "Dirty" Harry Callahan.  The film drew upon the actual case of the Zodiac Killer as the Callahan character seeks out a similar vicious psychopath.

Dirty Harry (character): Inspector Harold Francis "Dirty Harry" Callahan is a fictional character in the "Dirty Harry" film series, encompassing "Dirty Harry" (1971), "Magnum Force" (1973), "The Enforcer" (1976), "Sudden Impact" (1983) and "The Dead Pool" (1988).  Callahan is portrayed by Clint Eastwood in each film.

Magnum Force: Magnum Force is a 1973 American action thriller and the second to feature Clint Eastwood as maverick cop Harry Callahan after the 1971 film "Dirty Harry".  Ted Post, who also directed Eastwood in the television series "Rawhide" and the feature film "Hang 'Em High", directed this second installment in the "Dirty Harry" film series.  The screenplay was written by John Milius (who provided an uncredited rewrite for the original film) and Michael Cimino.  The film score was once again composed by Lalo Schifrin.  This film features early appearances by David Soul, Tim Matheson and Robert Urich.  At 124 minutes, it is also the longest "Dirty Harry" film.

David Gilliam: David Gilliam is a Canadian born film and television actor, who first appeared on screen as Alice in popular Clint Eastwood film Dirty Harry.

Zodiac Killer: The Zodiac Killer or Zodiac was a serial killer who operated in northern California in the late 1960s and early 1970s.  The killer's identity remains unknown.  The Zodiac murdered victims in Benicia, Vallejo, Lake Berryessa, and San Francisco between December 1968 and October 1969.  Four men and three women between the ages of 16 and 29 were targeted.  The killer originated the name "Zodiac" in a series of taunting letters sent to the local Bay Area press.  These letters included four cryptograms (or ciphers).  Of the four cryptograms sent, only one has been definitively solved.

Supporting paragraphs: Dirty Harry Zodiac Killer
Answer: December 1968 and October 1969