you will have to run each part of this seperately to see what is going on and debug it.
I assumed that you have a format on the process that inserts the data into the memo field so I went with your sample memo field.
create a formula field as
stringvar t1;
WhileReadingRecords;
t1 := mid({table.memofield},instrrev({table.memofield},"> Sent: ")+8);
t1
that should return a string that starts with the date field you wanted to extract (but also includes the whole notation).
If that is correct move to the next part and see what t2 returns. It should trim off the everything right of the AM or PM on the date field.