0.5 years after his ostensible birth is a date within that first year: 30 June 1AD.
1.5 years after his birth is not within that “first year”; it is within the “second year”, or 2AD. Just like 1.5" is a measurement that falls within the “second inch” of a ruler.
Is 0.5" within the “first inch” of a ruler?
1AD is every date within the “first year”.
0.5 years after his ostensible birth is a date within that first year: 30 June 1AD.
1.5 years after his birth is not within that “first year”; it is within the “second year”, or 2AD. Just like 1.5" is a measurement that falls within the “second inch” of a ruler.