title: Convert a future local DateTime into UTC
body: <p>I'm facing an issue converting a  EWS Appointment's start <code>DateTime</code> from the local time into UTC. From EWS, I get the Appointment's <code>DateTime</code> and <code>TimeZone</code>. I have been using <code>.ToUniversalTime()</code> to convert the <code>DateTime</code> to UTC. </p>
  
  <p>However, when the current time and the Appointment's start time are on different sides of daylight savings time (for example, the start <code>DateTime</code> might be March 15, 2018), the <code>.ToUniversalTime()</code> conversion uses the <em>current</em> UTC offset, which results in a DateTime that is off by one hour. </p>
  
  <p>I need some way of getting the UTC offset at a future date. </p>
  
  <p>My best plan is to use <code>.ToUniversalTime()</code> to get an approximation of the correct UTC start <code>DateTime</code>. Most of the time, this will be the correct UTC conversion, and when it's not, it'll be close. Theoretically, I should be able to get a predicted UTC offset at this estimated UTC time, and reconvert the original start DateTime to UTC. </p>
  
  <p>I'm aware this will fail if the start <code>DateTime</code> occurs within an hour after daylight savings time switches, but that's close enough for me. I would appreciate any suggestions or alternate methods, but what I'm really searching for is some way of getting the estimated UTC offset at some future date. </p>