Fix IndexType to use AbstractDateTime rather than TimeType

[fsaad]

The Dates module has a problem

julia > using Dates
julia> t = Time("14:20")
julia> DateTime(t)
ERROR: MethodError: Cannot `convert` an object of type Time to an object of type DateTime

Closest candidates are:
  convert(::Type{DateTime}, ::Date)
   @ Dates ~/julia-1.9.3/share/julia/stdlib/v1.9/Dates/src/conversions.jl:30
  convert(::Type{DateTime}, ::Millisecond)
   @ Dates ~/julia-1.9.3/share/julia/stdlib/v1.9/Dates/src/conversions.jl:34
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:64
  ...

Stacktrace:
 [1] DateTime(dt::Time)
   @ Dates ~/julia-1.9.3/share/julia/stdlib/v1.9/Dates/src/conversions.jl:20

Even though any TimeType is allowed in DateTime:

julia> methods(DateTime)
...
[28] DateTime(dt::TimeType)
...