add operations

src/Periods.jl

@@ -5,4 +5,15 @@ abstract type AbstractPeriod end
 include("SimplePeriods.jl")
 export Frequency, Period, Year, Semester, Quarter, Month, Week, Day, Undated
 
+import Base.:-
+-(p1::AbstractPeriod, p2::AbstractPeriod) = p1.ordinal - p2.ordinal
+
+import Base.:>
+>(p1::AbstractPeriod, p2::AbstractPeriod) = p1.ordinal > p2.ordinal
+import Base.:<
+<(p1::AbstractPeriod, p2::AbstractPeriod) = p1.ordinal < p2.ordinal
+
+import Base.checkindex
+checkindex(::Type{Bool}, inds::AbstractUnitRange, p::AbstractPeriod) = checkindex(Bool, inds, p.ordinal)
+
 end # module

src/SimplePeriods.jl

@@ -46,3 +46,11 @@ end
 
 Period(year::Integer, month::Integer, day::Integer) = (Date(year, month, day).value, Day)
 Period(d::Date) = (d.value, Day)
+
+import Base.copy
+copy(p::Period) = Period(p.ordinal, p.frequency)
+
+import Base.:+
++(p1::Period, k::Integer) = Period(p1.ordinal + k, p1.frequency)
+import Base.:-
+-(p1::Period, k::Integer) = Period(p1.ordinal - k, p1.frequency)