change log level

api.go

diff --git a/api.go b/api.go
index e3406ab..287d782 100644
--- a/api.go
+++ b/api.go
@@ -415,6 +415,6 @@
 			return timestamp.Format(iso8601)
 		}
 	}
-	log.Panic("convertTime: Unsupported time format: " + t)
-	return ""
+	log.Error("convertTime: Unsupported time format: " + t)
+	return t
 }