Support for milliseconds

[Curetix]

Hi, I'd like to request the possibility to explicitly work with milliseconds during the various operations this library offers. The uppercase S is a common template token for parsing and formatting milliseconds, with the ability to vary the number of decimal places using multiple S (e.g. .S, .SS, .SSS).

Parsing a datetime with milliseconds already works when not using a custom format, for example parse("2022-08-03 16:56:54.477") returns the appropriate date object 2022-08-03T14:56:54.477Z.

Another thing I encountered is that tzDate seems to remove the milliseconds part of the date, both tzDate("2022-08-03 16:56:54.477", "Europe/Berlin") and tzDate(parse("2022-08-03 16:56:54.477"), "Europe/Berlin") return 2022-08-03T14:56:54.000Z.

[hrynevychroman]

I looked to this and need some help from OG devs inside this repo, so let's look into this.

Firstly, I see that typescript@5.3.3 interface DateTimeFormatPartTypesRegistry don't support fractionalSeconds at all:

Also I see that when we parsing date, function date setMilliseconds to zero, so I think this repo not configured to work with milliseconds at all 🥲

[cusxio]

It would have been helpful if this limitation was mentioned in the documentation. I only realized it after porting to this library midway through my app.

[tobychidi]

Yes milliseconds support is needed. Also diffMilliseconds seems to be broken.