Support Date/Time + Unitful arithmetic (#731)

Author: aplavin

src/dates.jl

@@ -188,3 +188,14 @@ isapprox(x::AbstractArray{Dates.CompoundPeriod}, y::AbstractArray{<:AbstractQuan
          kwargs...) = isapprox(y, x; kwargs...)
 
 sleep(x::AbstractQuantity) = sleep(ustrip(s, x))
+
+# Dates, Times, DateTimes
+
+for f in (:+, :-)
+    @eval Base.$f(x::Dates.DateTime, y::Quantity) = $f(x, trunc(Dates.Millisecond, y))
+    @eval Base.$f(x::Dates.Time, y::Quantity) = $f(x, trunc(Dates.Nanosecond, y))
+    @eval Base.$f(x::Dates.Date, y::Quantity) = $f(x, Dates.Day(y))
+end
+Base.:+(y::Quantity, x::Dates.DateTime) = x + y
+Base.:+(y::Quantity, x::Dates.Time) = x + y
+Base.:+(y::Quantity, x::Dates.Date) = x + y

test/dates.jl

@@ -31,6 +31,19 @@
             @test 1.0u"wk" - Day(3) === 1.0u"wk" - Int64(3)u"d"
             @test_throws DimensionError 1u"m" + Second(1)
             @test_throws DimensionError 1u"m" - Second(1)
+
+            @test Dates.Date(2000, 2, 3) + 24.0u"hr" == Dates.Date(2000, 2, 4)
+            @test Dates.Date(2000, 2, 3) + 48.0u"hr" == Dates.Date(2000, 2, 5)
+            @test_throws Exception Dates.Date(2000, 2, 3) + 1.0u"hr"
+
+            @test Dates.DateTime(2000, 2, 3, 4, 5) + 1.0u"ns" == Dates.DateTime(2000, 2, 3, 4, 5)
+            @test Dates.DateTime(2000, 2, 3, 4, 5) + 23.0u"hr" == Dates.DateTime(2000, 2, 4, 3, 5)
+            @test Dates.DateTime(2000, 2, 3, 4, 5) + 100.0u"hr" == Dates.DateTime(2000, 2, 7, 8, 5)
+            @test Dates.Time(4, 5) + 1.0u"ps" == Dates.Time(4, 5)
+            @test Dates.Time(4, 5) + 1.0u"ms" == Dates.Time(4, 5, 0, 1)
+            @test Dates.Time(4, 5) + 1.5u"hr" == Dates.Time(5, 35)
+            @test Dates.Date(2000, 2, 5) - 48.0u"hr" == Dates.Date(2000, 2, 3)
+            @test 24.0u"hr" + Dates.Date(2000, 2, 5) == Dates.Date(2000, 2, 6)
         end
 
         @testset ">> Multiplication" begin