fix stack overflow

Author: baumgold

stdlib/Dates/src/arithmetic.jl

@@ -7,6 +7,7 @@
 # TimeType arithmetic
 (+)(x::TimeType) = x
 (-)(x::T, y::T) where {T<:TimeType} = 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