Support for specifying valid date/time formats

It would be really helpful if there was a way to provide the **valid, accepted** date/time formats for parameter of type `DateTime`. Right now, some formats work - others don't - and I don't see much support for specifying which ones I'd like to accept - meaning in the end I need to use `string` as datatype and do the parsing myself (suboptimal).....

Any hope a feature like that might be added sooner or later? For a parameter of type `DateTime`, allow a `DateTimeFormat` attribute or some other way of specifying the accepted, valid date format strings.

Thanks!

The issue names no files, tests, or entry points. Start by locating how DateTime command-line parameters are parsed, then determine how a DateTimeFormat attribute or equivalent should specify accepted formats and how validation should behave; done means callers can provide formats without falling back to string parsing.

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