title: get timezone offset of another timezone in javascript without using Strings
body: <p>I want to calculate the offset from 'users time' to 'WET/WEST'.</p>
  
  <p>I get the users offset with new <code>Date().getTimezoneOffset()</code>.</p>
  
  <p>But how do I get the offset for WET/WEST So that I can calcluate the combined offset of both?</p>
  
  <p>For example, if the user is in central europe time (CET/CEST), in winter the combined offset would be -60 (CET) + 0 (WET) = -60. In summer, it would be -120 (CEST) + 60 (WEST) = -60. In this case it is always -60 but the user could also have a timezone without DST.</p>
  
  <p>Is this possible without format it to a string and read out the new timezone from that string?</p>