When did the time AD start?

The beginning of the year AD 1 immediately followed the end of the year 1 BC.

However at that time the BC/AD dating system did not exist, by the Roman calendar in use at the time 1 BC was the year 753 and AD 1 was the year 754.

The BC/AD dating system was devised in AD 525, but was not widely used until after AD 800. As originally intended Jesus Christ was supposed to have been born sometime in the year AD 1, but due to misconceptions and unreliable information on the time period (not surprising as his family was insignificant common people of whom official secular records were not kept and all Jewish Temple genealogical records had been burned by the Romans in AD 70), modern calculations estimate that Jesus Christ must have been born in 4 BC or earlier.