Following Yarowsky (1997), we use equivalence classes for context tokens classified as instances of specific semiotic classes such as numbers, currencies, measure expressions, or letter sequences; see Ebden and Sproat (2014) for a full list.
the featural representation of "1993 to present" and "2017 to present" are identical, since 1993 and 2017 are both instances of the DATE semiotic class.