Error converting due to wrong date type with instanceOf

[plee61]

Before checking for instanceof, date needs to be converted to Date object
and need to be checked for valid date just incase date is a null or empty string

function strftime(sFormat, sDate) {
	if (typeof sFormat !== 'string') {
	  return '';
	}
	if (sDate == "") {return sDate;}
	let date = new Date(sDate);
	if (!(date instanceof Date  && !isNaN(date))) {
	  return sDate;
	}