Enable ZonedDateTime to parse its own string representation

[GHTaarn]

When a ZonedDateTime with a zero millisecond component is represented as a string, then the millisecond component is not shown in the String leading to parse errors if the String is parsed, e.g.:

julia> x=ZonedDateTime("2024-09-03T18:42:59.000+00:00")
2024-09-03T18:42:59+00:00

julia> parse(ZonedDateTime, string(x))
ERROR: ArgumentError: Unable to parse date time. Expected directive Delim(.) at char 20
Stacktrace:
 [1] macro expansion
   @ /snap/julia/104/share/julia/stdlib/v1.10/Dates/src/parse.jl:104 [inlined]
 [2] tryparsenext_core(str::String, pos::Int64, len::Int64, df::DateFormat{Symbol("yyyy-mm-ddTHH:MM:SS.ssszzz"), Tuple{…}}, raise::Bool)
   @ Dates /snap/julia/104/share/julia/stdlib/v1.10/Dates/src/parse.jl:38
 [3] macro expansion
   @ /snap/julia/104/share/julia/stdlib/v1.10/Dates/src/parse.jl:150 [inlined]
 [4] tryparsenext_internal
   @ /snap/julia/104/share/julia/stdlib/v1.10/Dates/src/parse.jl:125 [inlined]
 [5] parse(::Type{ZonedDateTime}, str::String, df::DateFormat{Symbol("yyyy-mm-ddTHH:MM:SS.ssszzz"), Tuple{…}})
   @ Dates /snap/julia/104/share/julia/stdlib/v1.10/Dates/src/parse.jl:284
 [6] parse(::Type{ZonedDateTime}, str::String)
   @ Dates /snap/julia/104/share/julia/stdlib/v1.10/Dates/src/parse.jl:282
 [7] top-level scope
   @ REPL[228]:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> 

As far as I can tell, DateTime from the Dates standard library is always able to parse its own output and in my opinion, it would be nice if ZonedDateTime could do this as well.

[omus]

As far as I can tell, DateTime from the Dates standard library is always able to parse its own output and in my opinion, it would be nice if ZonedDateTime could do this as well.

I agree with this sentiment. TimeZones.jl uses Dates.jl for the underlying parsing engine and this will need to be addressed there to handle optional milliseconds with additional info after.

[GHTaarn]

Hello,
first of all, sorry for the long reply time, I have been very busy with other tasks until now.

I am glad that you agree that it would be desirable for ZonedDateTime to be able to parse its own String output,
but I am not so optimistic that Dates.TimeFormat would receive such an amendment any time soon. In my opinion,
there are many other satisfactory ways that this can be achieved, I have submitted a PR with a proposed solution, I would be
glad to get your comments on it.