In the notes for this unit there is a list of all the ordinals used in dates.
They are based on the Vigesimal system and numbers after 'ten' deg are the number on ten.
eg 11 = Un ar ddeg ; 11th = Unfed ar ddeg
13 = Tri ar ddeg; 13th = Trydydd ar ddeg
Things get a bit odd after 14 since:-
15 = Pymtheg (which is actually a contraction of 'Pump ar ddeg); 15th = Pymthegfed
16 = 'One on fifteen' = Un ar bymtheg; 16th = Unfed ar bymtheg
17 and 19 follow the pattern of 16
18, however is Deunaw (two nines); 18th = Deunawfed
This all sounds a bit archaic and complicated. Its origins are no doubt in the significance of certain numbers to the agricultural economies of Wales that existed for many hundreds of years before the industrial revolution.
It's possible the ordinals will follow the cardinal numbers to a simple decimal system in the near future.
Since adding (-fed, or -ed) to the appropriate decimal number is already creeping into even official uses.
Thanks - it was the use of 'ar' which I normally take to mean 'on' that confused me, but evidently it's a remnant of older usage. I would have expected a simple 'a' ('and'). Oh well....