title: math for calculating timezone offset at particular instance of utc time since epoch
body: <p>Is it possible to find (via A mathematical formula) a timezone (or offset) with a given time instance (for example in UTC) measures in millisecond/microseconds etc since January 1, 1970.</p>
  <p><code>result = getTimeZoneOffset(date.now(), some_Date_before_Or_After_The_date_now);</code></p>
  <p>how can I fill this method?</p>
  <p>For example: In dart language, we can only get offset relative to current timezone and not for others. Like, If I go back in UTC time what will be the offset at that particular time instance.
  Please advise.</p>
  <p><em>P.S. I hope, I am not confusing stuff here.</em></p>