I'm sure there's a simpler way of doing this but this might help until you find something better.
Create 2 formulas and append together.
Formula 1
if(day({yourdatefield}))in [1,21,31] then 'st'
if(day({yourdatefield}))in [2,22] then 'nd' else
if(day({yourdatefield}))in [3,23] then 'rd' else 'th'
Formula 2
cstr(day({yourdatefield})+{formula1name}