I think you're laboring under a misconception... both BC and AD are associated with the birth of Yeshua rather than as you suppose. There are no intervening years... AD doesn't mean after death... it's from the Latin for Anno Domini In the Year of our Lord...
Caesar wouldn't have said 55BC was 55BC. It would have been something entirely different to him. This from the internet.
In the early days, Romans denoted years by the names of the two Consuls who ruled each year and that system continued long after other ways of denoting the year were used. Later they began to count the years from the foundation of the City of Rome. There is no single agreed date for that but a Roman writer Marcus Terentius Varro fixed the date as what we would call 753BC.