ConstReadonlyDay: "d"A calendar day.
ReadonlyMonth: "m"A calendar month, the day-of-month carried across it.
ReadonlyWeekday: "wd"A business day — Saturday and Sunday are skipped.
ReadonlyYear: "y"A calendar year.
Weekday is a BUSINESS day: Saturday and Sunday are not units of it, so Fri plus one weekday
is the following Monday.
IAutoFill.DateSeries, which reports the unit a template turns out to hold.
The unit a date step counts — what makes one date series a different series from another.