FieldKind = enum
  fkYear, fkMonth, fkDayOfMonth, fkDayOfWeek, fkHour, fkMinute, fkSecond

Field = ref object
  kind*: FieldKind
  expr*: Expr

proc minValue(field: Field; dt: DateTime): int {...}{.raises: [], tags: [].}

proc maxValue(field: Field; dt: DateTime): int {...}{.raises: [], tags: [].}

proc getValue(field: Field; dt: DateTime): int {...}{.raises: [], tags: [].}