If you have the date in a text format (e.g. loaded from a text file) then you first need to convert the text to a number using the date#() -function:
date#( <field>, 'DD-MMM-YYYY' ).
The MMM format code will match the three letter month names as defined in your MonthNames variable, e.g.:
If you in a second step want to format the date in a different way, you should use the date() -function outside the date#() -function:
date( date#( <field>, 'DD-MMM-YYYY' ), 'DD/MM/YY' ).