Ingests a six-character `YYYYMM` value (`"201903"`), or one already hyphenated (`"2019-03"`), and returns the canonical `"YYYY-MM"` form. `NA` is returned unchanged.
Ingests a six-character `YYYYMM` value (`"201903"`), or one already hyphenated (`"2019-03"`), and returns the canonical `"YYYY-MM"` form. `NA` is returned unchanged.