þritigoða [OE]
thritithe [MI]
thirtieth [ME]
þertieþ
https://www.etymonline.com/word/thirtieth#etymonline_v_39172
https://www.merriam-webster.com/dictionary/thirtieth
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED45536
https://bosworthtoller.com/32122