function getTimezoneOffsetInMilliseconds(dirtyDate) {
  var date = new Date(dirtyDate.getTime());
  var baseTimezoneOffset = Math.ceil(date.getTimezoneOffset());
  date.setSeconds(0, 0);
  var hasNegativeUTCOffset = baseTimezoneOffset > 0;
  var millisecondsPartOfTimezoneOffset = hasNegativeUTCOffset ? (MILLISECONDS_IN_MINUTE + getDateMillisecondsPart(date)) % MILLISECONDS_IN_MINUTE : getDateMillisecondsPart(date);
  return baseTimezoneOffset * MILLISECONDS_IN_MINUTE + millisecondsPartOfTimezoneOffset;
}
Google Chrome as of 67.0.3396.87 introduced timezones with offset that includes seconds.
They usually appear for dates that denote time before the timezones were introduced
(e.g. for 'Europe/Prague' timezone the offset is GMT+00:57:44 before 1 October 1891
and GMT+01:00:00 after that date)

Date#getTimezoneOffset returns the offset in minutes and would return 57 for the example above,
which would lead to incorrect calculations.

This function returns the timezone offset in milliseconds that takes seconds in account.