update

src/Periods.jl

 module Periods
 
+abstract type AbstractPeriod end
+
 include("SimplePeriods.jl")
+export Period
 
 end # module

src/SimplePeriods.jl

 @enum Frequency Year Semester Quarter Month Week Business Day
 
-struct SimplePeriod
+struct Period
     ordinal::Int64
     frequency::Frequency
 end
 
-function SimplePeriod(year, subperiod, frequency)
+function Period(year, subperiod, frequency) <: AbstractPeriod
     if frequency == Year
         ordinal = year - 1970
     elseif frequency == Semester