Original sentence: This numbering system lacks a year zero, which is a problem with some calculations; so in astronomical year numbering (which for the current era is based on the Christian numbering) a zero is added, and the 'AD' and 'BC' are dropped
Edited sentence: The ancient numbering system lacks a year zero, which is a problem with some calculations; so in astronomical year numbering (which for the current era is based on the Christian numbering) a zero is added, and the 'AD' and 'BC' are dropped (in common modern usage of AD, the year 0 is accepted)