Timeline...
Start of 1 BC..........End of one year|0|Start of 1 AD......end of year, Jan. 1 2AD|

Total time, two years. You don't count the zero! There is only a start and end of 1 AD with the zero representing the instant we end 1 BC and start counting 1 AD. You can't count 1 AD as completed until the year has ended.