From what I’m seeing you can use dd-MMM-yy, for example, Date.now().toLocaleDateString({“format”:“dd-MMM-yy”}) returns “27-Jul-20”.
I don’t think you Timestamp value is in one of the standard formats that we typically parse (See Parsing a Date on the Expression Language Examples page of our documentation), so you may need to follow the parsing a non-standard example of Date.parse($NonStandardDate, “dd-MMM-yy”)
Is Timestamp showing as a date-time field or a string? If it’s being treated as a string, then some other things will need to be done first.