FiscalYear constructor
Constructs a FiscalYear, optionally setting the month offset (which defaults to minus 3).
Implementation
FiscalYear(int year, {int monthOffset = -3})
: _year = year,
super(TimeInstant.dateTime(DateTime(yr4(year), 1 + monthOffset)),
TimeInstant.dateTime(DateTime(yr4(year), 13 + monthOffset)));