Refactor TradingHours struct and validation

date_utils/date_utils.go

@@ -458,7 +458,7 @@ type TradingHoursDay struct {
 }
 
 func (day TradingHoursDay) String() string {
-	if day.StartTime == "" && day.EndTime == "" {
+	if day.StartTime == "" || day.EndTime == "" {
 		return "Closed"
 	}