title: Convert milliseconds to Firestore date
body: <p>In Flutter I convert dates to milliseconds like this:</p>
  <pre><code>static DateTime createDateTimeNow() {
      return DateTime(DateTime.now().year, DateTime.now().month,
          DateTime.now().day, 0, 0, 0, 0, 0);
    }
  
  // This create a number value like 1660341600000
  final Number millisecondsNow = Helpers.createDateTimeNow().toUtc().millisecondsSinceEpoch;
  
  // I can also format is as String to be presented based on the language
  final String dateNow = DateFormat.yMMMMd(language).format(millisecondsNow);
  </code></pre>
  <p>Now I have Firebase Cloud Functions that should read these dates and send notification when they expire, but could I convert this number to a normal date in Firestore?</p>
  <p>From Flutter I do something like this:</p>
  <pre><code>final DateTime dateNow = DateTime.fromMillisecondsSinceEpoch(1660341600000);
  </code></pre>