I don’t think all the math is right here. A billion seconds is like 31 years, not 64.
I don’t write them, I just plagiarize.
You are right.
This site makes a conversion easy:
https://converths.com/how-many-years-is-a-billion-seconds-billion-sec-to-years/
1000000000 minutes = 1901.29 years so 121 AD almost 122AD