Bobby was born just before midnight on the 29th February and Billy born after midnight on the 1st March, So after 4 years Bobby is 1 and Billy is 4 therefore older.
wow, never thought of that way before... how do they celebrate people with feb. 29 as a birthday :-? well, the answer is that their mother gave birth to them in a fast plane/boat, and they crossed the international dateline in between the birth of the twins.
Of course the moment he came out his mother sooner he is technically by law and logic older. The only way you could twist it with good argument is when somehow you'd be able to confirm that his egg was fertilized earlier, or both his seed and egg were created earlier. But that's not quite possible.
Also that is not the moment from which age is measured. When you are born you are not proclaimed nine months old (even though technically you are).
Billy is older by law, for example, Bobby was born on 12:01 am, March 2nd. They crossed the International Dateline, in which the current date is still March 1st, 11:50 pm. and Billy was born on that time. So Bobby was born first, on 12:01 am, March 2 and Billy was born second, on 11:50 pm, March 1. I doubt this will happen though, unless there is some nerd on-board which proudly exclaims this.