Refactor to accommodate blank & invalid dates & times

Remove hour & min components in previous commit fecc88db57677869ff23330f3cf5a55d3908c3bb (as they are not currently required)

Adding support to allow string representations of dates/time.

This is to support a non dropdown web form where a user can supply
a date and time in separate input fields, if an error was to occur,
the user input should be represented to the user.

TODO:
* finish tests
* Update version number and documentation.

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@figwit @bendilley 
I don't think `%Z` works well when parsing - I think it's for formatting only. Have you tried `Time.zone.strptime`?