diff --git a/htdocs/core/lib/functions.lib.php b/htdocs/core/lib/functions.lib.php
index 0786ea6db3c..637bbdb8743 100644
--- a/htdocs/core/lib/functions.lib.php
+++ b/htdocs/core/lib/functions.lib.php
@@ -234,6 +234,25 @@ function isModEnabled($module)
 	//return !empty($conf->$module->enabled);
 }
 
+/**
+ * isDolTms check if a timestamp is valid.
+ *
+ * @param  int|string|null $timestamp timestamp to check
+ * @return bool
+ */
+function isDolTms($timestamp)
+{
+	if ($timestamp === '') {
+		dol_syslog('Using empty string for a timestamp is deprecated, prefer use of null when calling page '.$_SERVER["PHP_SELF"], LOG_NOTICE);
+		return false;
+	}
+	if (is_null($timestamp) || !is_numeric($timestamp)) {
+		return false;
+	}
+
+	return true;
+}
+
 /**
  * Return a DoliDB instance (database handler).
  *
diff --git a/htdocs/projet/class/project.class.php b/htdocs/projet/class/project.class.php
index a7fed3246dc..90a7d140341 100644
--- a/htdocs/projet/class/project.class.php
+++ b/htdocs/projet/class/project.class.php
@@ -849,9 +849,9 @@ class Project extends CommonObject
 			$sql = "SELECT rowid FROM ".MAIN_DB_PREFIX.$tablename." WHERE ".$projectkey." IN (".$this->db->sanitize($ids).") AND entity IN (".getEntity($type).")";
 		}
 
-		if ($date_start > 0 && $type == 'loan') {
+		if (isDolTms($date_start) && $type == 'loan') {
 			$sql .= " AND (dateend > '".$this->db->idate($date_start)."' OR dateend IS NULL)";
-		} elseif ($date_start > 0 && ($type != 'project_task')) {	// For table project_taks, we want the filter on date apply on project_time_spent table
+		} elseif (isDolTms($date_start) && ($type != 'project_task')) {	// For table project_taks, we want the filter on date apply on project_time_spent table
 			if (empty($datefieldname) && !empty($this->table_element_date)) {
 				$datefieldname = $this->table_element_date;
 			}
@@ -861,9 +861,9 @@ class Project extends CommonObject
 			$sql .= " AND (".$datefieldname." >= '".$this->db->idate($date_start)."' OR ".$datefieldname." IS NULL)";
 		}
 
-		if ($date_end > 0 && $type == 'loan') {
+		if (isDolTms($date_end) && $type == 'loan') {
 			$sql .= " AND (datestart < '".$this->db->idate($date_end)."' OR datestart IS NULL)";
-		} elseif ($date_end > 0 && ($type != 'project_task')) {	// For table project_taks, we want the filter on date apply on project_time_spent table
+		} elseif (isDolTms($date_end) && ($type != 'project_task')) {	// For table project_taks, we want the filter on date apply on project_time_spent table
 			if (empty($datefieldname) && !empty($this->table_element_date)) {
 				$datefieldname = $this->table_element_date;
 			}