Formats datetimes for compatibility with Excel
@memberof TableExport.prototype
@param v {Number}
@param date1904 {Date}
@returns {Number} epoch time
function(v, date1904) {
      if (date1904) v += 1462;
      var epoch = Date.parse(v);
      var result = (epoch - new Date(Date.UTC(1899, 11, 30))) / (24 * 60 * 60 * 1000);
      return Math.floor(result);
    }