A more straightforward explanation is that George Lucas, in writing the script, simply did not remember or realize that parsecs are not time measurements. According to some sources, Lucas did realize this, but wished to portray Han Solo as an absent-minded braggart who simply did not know what he was talking about. The bemused reactions of Obi-Wan Kenobi and Luke Skywalker to Han’s quip suggest that this could be the case.
Han Solo may be an absent-minded braggart but if there is one thing he knows, it’s the Millennium Falcon (and Sabacc). If Solo boasts about the Kessel run, expecting to impress Luke and Ben, I highly doubt he would use a completely wrong increment of measure to do so. This guy knows how to navigate hyperspace and avoid passing right through stars or bouncing to near supernovas. I am sure he knows the correct use of the term parsec.
Fans often forget that before their demi-gods were such they were normal people who drank coffee, used the bathroom, and made mistakes. (In some cases they still do *cough* Greedo shooting first *cough*) We will look to the script drafts on Starkiller: The Jedi Bendu Script Site for reference. It is not until the fourth draft of Star Wars that the scene in question appears:
HAN
Fast ship? You’ve never heard
of the Millennium Falcon?
BEN
Should I have?
HAN
It’s the ship that made the Kessel
run in less than twelve parsecs!
Ben reacts to Solo’s stupid attempt to impress them with obvious misinformation.
HAN
(continuing)
I’ve outrun Imperial starships,
not the local bulk-cruisers, mind
you. I’m talking about the big
Corellian ships now. She’s fast
enough for you, old man. What’s
the cargo?
Problem solved right? There it is, right in the script by Lucas himself! Not necessarily. Note on the Starkiller site that this fourth draft was published as:
Star Wars, Episode IV: A New Hope, from the Journal of the Whills. Public Version of Fourth Draft. As published in The Art of Star Wars, Ballantine 1979. Dated: “Revised Fourth Draft, January 15, 1976.”
Although it claims to have been written a year before the film was released, this script didn’t appear publicly until two years after. Here is another revised version of the fourth draft dated March 15, 1976:
HAN
Fast ship! You mean you’ve never heard of the Millennium Falcon?
BEN
(amused)
Should I?
HAN
It’s the ship that made the Kessel run in less than 12 par-sec’s! I’ve outrun Imperial starships, not local bulk-cruisers mind you. These are the big Corellian ships I’m talking about. I think she’s fast enough for you old man. What’s your cargo?
BEN
No questions. Is it local trouble?
BEN
Let’s just say we’d like to avoid any Imperial entanglements.
Is it possible Ben’s reaction was simply added as an afterthought for the 1979 public release of the film’s script. The answer isn’t definite… but at the risk of sounding too much like a fanboy I would say the evidence against Lucas’ divine perfection is a little too heavy to ignore.
Nick: What is your writing process like? Are the songs written primarily together as a band or do you separately work on tracks and then bring them to the table?
They were just making fun of 
Yes, yes, this is just another sad blog post relating some cultural phenomena to the passing of youth. All the hippest bloggers do it, so sue me. But today marks a new day in the history of Nick Martinolich. Today is the day I crack open my unfinished copy of Tales Of The Bounty Hunter. Tomorrow is the day Jimmie and I once again discuss the absurdness of scout troopers having no radio contact with the Imperial base on the 
Transcribed from 
-The recording of our EP is complete.
Once I learned of a man named