Function used to convert a numeric value (from Excel or Google Sheets) into a JavaScript Date object.

fromExcelDate.js

/**
 * Converts a numeric value (from Excel or Google Sheets) into a JavaScript Date object.
 * @param {number} numDate - The numeric value that will be converted into a date.
 * @param {?number=} opt_tzMinOffset - Optional.  Defaults to the minute offset of the
 *     local timezone.  If given this should indicate how many minutes to offset the date
 *     by to correctly represent the desired timezone.
 * @returns {Date} - The date representation of the numeric value passed in.
 */
function fromExcelDate(num, opt_tzMinOffset) {
  return new Date(
    (num - 25569) * 864e5
    + (opt_tzMinOffset !== undefined ? -opt_tzMinOffset || 0 : (new Date).getTimezoneOffset()) * 6e4
  );
}