Convert 1/2/2021 to 2021-2-1

I have a string that comes in the format name = "1/2/2021", how can I convert this string to the format newName = "2021-2-1"

original =  name  day = original[0]  month = original[2]  year = original[4:8]  combination = year + "-" + month + "-" + day  

I have tried using this but brings wrong values when date or month changes to more than 2 characters

https://stackoverflow.com/questions/65546132/convert-1-2-2021-to-2021-2-1 January 03, 2021 at 10:30AM