title: Store date with optional month / day
body: <p>I want to store date in my Postgres database.<br>
  The only problem is that this date can have optional day or even month.</p>
  
  <p>Example:<br>
  User provides time period when he was employed - not necessary full date (day + month + year), but only start year and end year.<br>
  However there are users, who worked only from may to october in the same year so month have to be provided too.</p>
  
  <p>How to handle this kind of optional date parts?</p>