Fix milestone deadline and date related problems (#32339)

Use zero instead of 9999-12-31 for deadline
Fix #32291

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

routers/common/deadline.go

@@ -0,0 +1,31 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package common
+
+import (
+	"time"
+
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/timeutil"
+)
+
+func ParseDeadlineDateToEndOfDay(date string) (timeutil.TimeStamp, error) {
+	if date == "" {
+		return 0, nil
+	}
+	deadline, err := time.ParseInLocation("2006-01-02", date, setting.DefaultUILocation)
+	if err != nil {
+		return 0, err
+	}
+	deadline = time.Date(deadline.Year(), deadline.Month(), deadline.Day(), 23, 59, 59, 0, deadline.Location())
+	return timeutil.TimeStamp(deadline.Unix()), nil
+}
+
+func ParseAPIDeadlineToEndOfDay(t *time.Time) (timeutil.TimeStamp, error) {
+	if t == nil || t.IsZero() || t.Unix() == 0 {
+		return 0, nil
+	}
+	deadline := time.Date(t.Year(), t.Month(), t.Day(), 23, 59, 59, 0, setting.DefaultUILocation)
+	return timeutil.TimeStamp(deadline.Unix()), nil
+}