The calendar systems used by Klingons have been mentioned a few times in canon episodes, and even more detailed information has been given in several novels. There appears to have been two separate methods used for reckoning dates in the Empire though. Both have been used in canon and in Trek Literature. Both these systems will be examined in greater detail here.
Several attempts have been made by others to examine a Klingon calendar system elsewhere. These are linked to the right. It was my desire to stay consistent with any of the info suggested on these other sites possible, but neither system is reconcilable with the exact dates mentioned in the Litverse. The majority of the basis for the primary and most detailed of the two Klingon calendar systems though, originated elsewhere.
I will call this system the Kenson-Bennett Klingon Calendar. A bit of history...
In 1999, Steve Kenson wrote up an extensive article on Klingons for an RPG book to be called "Blood and Honor" for Last Unicorn Games' Star Trek RPG. That book was, in the end,never published, but Steve later released his draft online. After some time his original document was removed from his site, but copies of it (or parts of it) have been posted on various sites since.
By 2007, Michael A Martin and Andy Mangels had published Excelsior: Forged in Fire, which with the help of Keith R. A. DeCandido had included reference to several of the months originally coined by Steve Kenson, and had assigned very specific Earth dates to Klingon dates. The exact story of who figured out these date conversions is lost to time, but Martin and Mangels continued to use this system in their Enterprise novels in the following years.
By 2011, Christopher L. Bennett had taken these month names, date conversions, and what little else he could glean and created a much more detailed analysis of this Klingon calendar system. He added a ninth month, originally mentioned in canon after Steve had written his article. And he figured out alot of calculations to make the system work. All of this was done in order to create two dates mentioned in his novel DTI: Watching the Clock.
So besides the definite contributions by Martin, Mangels, and DeCandidio, the two main documents I can site as the origin of the Kenson-Bennett Klingon Calendar are these:
I contacted Steve while trying to figure out the origin of this calendar, and he was both shocked and pleasantly surprised to hear that this material was still being read and that it had ended up in some Star Trek novels. He gave me permission to post this Klingon History document here.
I took what Christopher had given in his Calendar Notes, and did ALOT of math (and I mean ALOT), made a few tweaks to the small details, and created an explanation of ten steps you'd need to do to convert any date between the Earth and Klingon calendars. Then I analyzed all the mentions of Klingon dates I could find in episodes, novels, and comics.
Then there is the other Klingon dating system, which was also mentioned in an episode and several novels. This seems to be a totally different way of reckoning the Klingon year. Each example of its' use seems to be in relation to logs given by members of the Klingon Defense Force, and as such I theorize it is similar to the Stardate system used by Starfleet as opposed to the Gregorian calendar used for common Earth dates. I've analyzed it as much as I could.
Add to all this data a fair amount of extrapolations I've made concerning holidays, birth dates, and the Klingon military, and you'll see everything I've been able to figure out about the Klingon Calendar. I hope this can be of help to someone, be they a fan or even a writer.