Mark99

I am transcribing an old diary that includes many dates written as abbreviated ordinal numbers, e.g., 1stn 2tn 3tn, etc. I think this is perhaps archaic or non-standard. Unfortunately some of the abbreviations are fairly illegible. I'm not sure if the twenty first through the thirty-first days are abbreviated 21stn, 22stn..., or 21tn, 22tn.... Is there a rule for this?