fixup to #51743, timetype subtraction

Restores the method whose removal was probably causing problems.

(cherry picked from commit f6f1ee9)

Author: JeffBezanson

stdlib/Dates/test/arithmetic.jl

@@ -11,8 +11,13 @@ using Dates
     @test Dates.CompoundPeriod(a - b) == Dates.Hour(12)
 end
 
+struct MonthlyDate <: TimeType
+    instant::Dates.UTInstant{Month}
+end
 @testset "TimeType arithmetic" begin
     @test_throws MethodError DateTime(2023, 5, 2) - Date(2023, 5, 1)
+    # check that - between two same-type TimeTypes works by default
+    @test MonthlyDate(Dates.UTInstant(Month(10))) - MonthlyDate(Dates.UTInstant(Month(1))) == Month(9)
 end
 
 @testset "Wrapping arithmetic for Months" begin