Annulus is also related to annual, or the yearly cycle. Related to anno domini - year of our lord. The double 'n' seems to be correct historically although they have a common origin.

From what i understand of the situation: The era designator 'ad' is an abbreviation for 'anno domini', meaning 'in the year of the lord'. By that reasoning, 'ad 1453' is a complete dependent clause. For example, the gregorian calendar has bc and ad, meaning before christ and anno domini, the year of our lord. Or, if you are secular or follow a different religion, bce (before common era) and … Here are just a few problems with bc/ad: They're inconsistent.

Or, if you are secular or follow a different religion, bce (before common era) and … Here are just a few problems with bc/ad: They're inconsistent. Bc is an abbreviation of the english phrase before christ, while ad is an abbreviation of a latin phrase anno domini. It's very strange …