Add test and note for Dates.days(::Year)

The average number of days in a year should be the same in both
Unitful and the Dates stdlib.

We also add a reference to this function in the documentation for this
unit.

docs/src/dates.md

@@ -12,7 +12,7 @@ end
 The `Dates.FixedPeriod` union type includes all `Dates.Period`s that represent a fixed period of time, i.e., `Dates.Week`, `Dates.Day`, `Dates.Hour`, `Dates.Minute`, `Dates.Second`, `Dates.Millisecond`, `Dates.Microsecond`, and `Dates.Nanosecond`. These types can be converted to `Quantity`s or used in place of them.
 
 !!! note
-    `Dates.Year` does not represent a fixed period and cannot be converted to a `Quantity`. While Unitful's `yr` unit is exactly equal to 365.2425 days, a `Dates.Year` may contain 365 or 366 days (see [`Dates.isleapyear`](@ref)).
+    `Dates.Year` does not represent a fixed period and cannot be converted to a `Quantity`. While Unitful's `yr` unit is exactly equal to 365.2425 days, a `Dates.Year` may contain 365 or 366 days (see [`Dates.days(::Year)`](@ref) and [`Dates.isleapyear`](@ref)).
 
 Each `FixedPeriod` is considered equivalent to a `Quantity`. For example, `Dates.Millisecond(5)` corresponds to the quantity `Int64(5)*u"ms"`. A `FixedPeriod` can be converted to the equivalent `Quantity` with a constructor:
 

test/dates.jl

@@ -206,6 +206,7 @@
             @test u"μs"(CompoundPeriod(Day(365),Hour(5),Minute(49),Second(12))) === Rational{Int64}(31_556_952_000_000,1)u"μs"
             @test uconvert(u"ns", CompoundPeriod(Day(365),Hour(5),Minute(49),Second(12))) === Int64(31_556_952_000_000_000)u"ns"
             @test u"ns"(CompoundPeriod(Day(365),Hour(5),Minute(49),Second(12))) === Int64(31_556_952_000_000_000)u"ns"
+            @test uconvert(u"ms", 1u"yr") == (Dates.toms(Year(1)))u"ms"  # Compare to Dates stdlib value of a year
             @test uconvert(u"ps", CompoundPeriod(Week(1),Hour(-1))) === Int64(601_200_000_000_000_000)u"ps"
             @test u"ps"(CompoundPeriod(Week(1),Hour(-1))) === Int64(601_200_000_000_000_000)u"ps"
             @test_throws DimensionError uconvert(u"m", CompoundPeriod(Day(365),Hour(5),Minute(49),Second(12)))