title: Use nulls for Date Range where Start Date or End Date might be unknown?
body: <p>I'm working on a project where we track a person's history with a company. The table has StartDate and EndDate columns.<br>
  We won't always know Start Date of a person's history record.<br>
  For a person's current employment record, End Date will obviously be unknown.</p>
  
  <p>My question: Is it better in regards to storage, performance, querying, general usability etc to...<br>
  <strong>a)</strong> Make both of these columns nullable?<br>
  <em>or</em><br>
  <strong>b)</strong> Make both columns NOT NULL and have StartDate be a min value (1/1/1753) when unknown and the EndDate a max value (12/31/9999) when unknown?</p>
  
  <p>I understand how to tell if a record is past, current or future with either approach.<br>
  I've read the articles on <a href="https://stackoverflow.com/questions/5618357/sql-server-null-vs-empty-string">Null vs Empty String</a>, but this is a bit different with it being two associated fields that are often queried together.</p>