diff --git a/xlsxwriter.class.php b/xlsxwriter.class.php
index cc8c15112..76868cf00 100644
--- a/xlsxwriter.class.php
+++ b/xlsxwriter.class.php
@@ -365,9 +365,19 @@ protected function writeCell(XLSXWriter_BuffererWriter &$file, $row_number, $col
 		} elseif (is_string($value) && $value{0}=='='){
 			$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="s"><f>'.self::xmlspecialchars($value).'</f></c>');
 		} elseif ($num_format_type=='n_date') {
-			$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="n"><v>'.intval(self::convert_date_time($value)).'</v></c>');
+			$dateValue = self::convert_date_time($value);
+			if ($dateValue > 0 || preg_match("/(\d+):(\d{2}):(\d{2})/", $value)) {
+				$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="n"><v>'.intval($dateValue).'</v></c>');
+			} else { //not date, treat it as string
+				$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="inlineStr"><is><t>'.self::xmlspecialchars($value).'</t></is></c>');
+			}
 		} elseif ($num_format_type=='n_datetime') {
-			$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="n"><v>'.self::convert_date_time($value).'</v></c>');
+			$dateValue = self::convert_date_time($value);
+			if ($dateValue > 0 || preg_match("/(\d+):(\d{2}):(\d{2})/", $value)) {
+				$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="n"><v>'.$dateValue.'</v></c>');
+			} else { //not datetime, treat it as string
+				$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="inlineStr"><is><t>'.self::xmlspecialchars($value).'</t></is></c>');
+			}
 		} elseif ($num_format_type=='n_numeric') {
 			if (!is_string($value) || $value=='0' || ($value[0]!='0' && ctype_digit($value)) || preg_match("/^\-?(0|[1-9][0-9]*)?(\.[0-9]+)?$/", $value)){
 				$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="n"><v>'.self::xmlspecialchars($value).'</v></c>');//int,float,currency