Namaz Before It's Time
The start time of any salah is not digital in nature like 0 or 1. It’s a duration. I personally think if you started the salah one minute before the Azan and the Azan happens when the Isha time on the calendar start, there is no need to repeat the salah. When there were no clocks, people used to estimate time for prayers by looking outside and that estimation used to fall within a specific duration. This is my personal opinion.