sixtieth
Concise Oxford English Dictionary © 2008 Oxford University Press:
sixty/ˈsɪksti/
▶cardinal number (pl. sixties) the number equivalent to the product of six and ten; ten more than fifty; 60. (Roman numeral: lx or LX)
– derivatives
sixtieth ordinal number,
sixtyfold adjective & adverb.
sixtieth ordinal number,
sixtyfold adjective & adverb.
'sixtieth' also found in these Oxford entries:

