Partial dates

A strategy for handling partial / fuzzy dates.

In the answer to [strategy-for-incomplete-dates](http://stackoverflow.com/questions/4301172/strategy-for-incomplete-dates)

> Store the date as an integer -- yyyymmdd.
>
> You can then zero out any month or day component that has not been entered
>
> Year only: 1954 => 19540000
> Year & Month: April 2004 => 20040400
> January 1st, 2011 => 20110101
> Of course I am assuming that you do not need to store any time of day information.
>
> You could then create a struct to encapsulate this logic with useful properties indicating which level of granularity has been set, the relevant System.DateTime, etc
>
> Edit: sorting should then work nicely as well

The issue names no source files, tests, or entry points; start by reviewing the linked strategy-for-incomplete-dates discussion and the repository's current date handling. The work is not yet bounded: a contribution would need agreed behavior for year-only, year-and-month, and full dates, including sorting.

Written by the indexing model from the issue text.