diff --git a/kernel/write.c b/kernel/write.c
index 9d4c35d..4adef44 100644
--- a/kernel/write.c
+++ b/kernel/write.c
@@ -249,9 +249,11 @@ void chart_writer(zend_long row, zend_long columns, xls_resource_chart_t *chart_
  */
 void datetime_writer(lxw_datetime *datetime, zend_long row, zend_long columns, zend_string *format, xls_resource_write_t *res, lxw_format *format_handle)
 {
-    if (format_handle != NULL) {
-        format_set_num_format(format_handle, ZSTR_VAL(format));
+    lxw_format *value_format = format_handle;
+    if (format_handle == NULL) {
+        value_format  = workbook_add_format(res->workbook);
     }
+    format_set_num_format(value_format, ZSTR_VAL(format));
 
     worksheet_write_datetime(res->worksheet, (lxw_row_t)row, (lxw_col_t)columns, datetime, format_handle);
 }