title: Start of day in a particular timezone
body: <p>How do I get the start of the day in a given timezone?</p>
  <p>For example, if I had a date object <code>x</code> representing Jan 1 9pm GMT, then <code>x.startOfDayInTimeZone(&quot;Australia/Perth&quot;)</code> should be Jan 1 4pm GMT, as &quot;Australia/Perth&quot; is GMT+8 (so Jan 1 9pm GMT is Jan 2 5am Perth, and the start of day is Jan 2 midnight Perth which is Jan 1 4pm GMT).</p>
  <p>I've already got the library <code>date-fns</code> and <code>date-fns-tz</code> installed so if I could do this using those libraries that would be preferred (but not essential).</p>