In Shingeki no Kyojin, they comment a number of times about what year it is. The show starts in year 845. Though the world seems to be relatively similar to ours (having Asia, etc.), the don't seem to use any sort of marker for the years (at least that I saw) such as AD, CE, The Year of Our Lord, etc.

Is there any indication or mention of what year zero (or one) of their calendar is, as well as what their calendar is based off of?