They switched when years are counted. BC years are counted at the beginning of the year. AD years are counted at the end of the year.

The halfway mark in the first inch of a ruler is 0.5". The first inch ends at 1.00. 1.25" falls in the second inch of the ruler.

We recently completed the 3rd month in the 2024th year AD. We have not completed the full, 2024th year yet; but we are in it. This most recent April 1st was 2023.25.

1-Jan-1 BC was almost a year before 31-Dec-1 BC. 31-Dec-1 AD was a year after 31-Dec-1 BC.