+@code{epoch} is the year that starts the epoch. A 2-digit year is
+interpreted as belonging to the 100 years beginning at the epoch. The
+default epoch year is 69 years prior to the current year; thus, in
+2017 this field by default contains 1948. In the corpus, @t{epoch}
+ranges from 1943 to 1948, plus some contain -1.
+