When you call the FormatCommandLine DateTime and DateTime? are not formatted correctly (if you feed the generated string back to the CommandLine it shows an error)

internal class Options
{
	[Option(
		longName: "start",
		Default = null,
		HelpText = "Some help text"
	)]
	public DateTime? Start { get; set; }
}

Formatted date looks something like this for me: 14/08/2019 09:51:12

There seem to be missing " quotations around it (or a T between the date and the time) also the formatting of the date is wrong, should be 2019-08-14