There are many ways to achieve the result you are looking. Here is one of the oldies, using RIGHT, LEN and FIND
=ArrayFormula(RIGHT(A:A,LEN(A:A) - FIND(" on ",A:A) - 3)
Another way, in this case using SPLIT, SUBSTITUTE and INDEX
=ArrayFormula(INDEX(SPLIT(SUBSTITUTE(A:A," on ","|"),"|"),1,2))
One way using regular expressions
=ArrayFormula(REGEXEXTRACT(A:A," on (.+)"))
P.S. The simplest way to avoid having errors for blanks cells below the las row having a value on A, delete all the bottom blank rows.