further fix to the new promoting method for AbstractDateTime subtraction (#51967)

Author: JeffBezanson

stdlib/Dates/src/arithmetic.jl

@@ -7,7 +7,7 @@
 # TimeType arithmetic
 (+)(x::TimeType) = x
 (-)(x::T, y::T) where {T<:TimeType} = x.instant - y.instant
-(-)(x::DateTime, y::DateTime) = x.instant - y.instant
+(-)(x::T, y::T) where {T<:AbstractDateTime} = x.instant - y.instant
 (-)(x::AbstractDateTime, y::AbstractDateTime) = -(promote(x, y)...)
 
 # Date-Time arithmetic

stdlib/Dates/test/arithmetic.jl

@@ -14,10 +14,15 @@ end
 struct MonthlyDate <: TimeType
     instant::Dates.UTInstant{Month}
 end
+struct OtherTime <: Dates.AbstractDateTime
+    instant::Dates.UTInstant{Nanosecond}
+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)
+    # ... and between two same-type AbstractDateTimes
+    @test OtherTime(Dates.UTInstant(Nanosecond(2))) - OtherTime(Dates.UTInstant(Nanosecond(1))) == Nanosecond(1)
 end
 
 @testset "Wrapping arithmetic for Months" begin