diff --git a/backend/internal/service/announcement.go b/backend/internal/service/announcement.go
index 25c66eb4..02741d37 100644
--- a/backend/internal/service/announcement.go
+++ b/backend/internal/service/announcement.go
@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/domain"
+	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
 )
 
@@ -34,8 +35,23 @@ const (
 )
 
 var (
-	ErrAnnouncementNotFound      = domain.ErrAnnouncementNotFound
-	ErrAnnouncementInvalidTarget = domain.ErrAnnouncementInvalidTarget
+	ErrAnnouncementNotFound        = domain.ErrAnnouncementNotFound
+	ErrAnnouncementInvalidTarget   = domain.ErrAnnouncementInvalidTarget
+	ErrAnnouncementNilInput        = infraerrors.BadRequest("ANNOUNCEMENT_INPUT_REQUIRED", "announcement input is required")
+	ErrAnnouncementInvalidTitle    = infraerrors.BadRequest("ANNOUNCEMENT_TITLE_INVALID", "announcement title is invalid")
+	ErrAnnouncementContentRequired = infraerrors.BadRequest(
+		"ANNOUNCEMENT_CONTENT_REQUIRED",
+		"announcement content is required",
+	)
+	ErrAnnouncementInvalidStatus     = infraerrors.BadRequest("ANNOUNCEMENT_STATUS_INVALID", "announcement status is invalid")
+	ErrAnnouncementInvalidNotifyMode = infraerrors.BadRequest(
+		"ANNOUNCEMENT_NOTIFY_MODE_INVALID",
+		"announcement notify_mode is invalid",
+	)
+	ErrAnnouncementInvalidSchedule = infraerrors.BadRequest(
+		"ANNOUNCEMENT_TIME_RANGE_INVALID",
+		"starts_at must be before ends_at",
+	)
 )
 
 type AnnouncementTargeting = domain.AnnouncementTargeting
diff --git a/backend/internal/service/announcement_service.go b/backend/internal/service/announcement_service.go
index c0a0681a..12479041 100644
--- a/backend/internal/service/announcement_service.go
+++ b/backend/internal/service/announcement_service.go
@@ -70,16 +70,16 @@ type AnnouncementUserReadStatus struct {
 
 func (s *AnnouncementService) Create(ctx context.Context, input *CreateAnnouncementInput) (*Announcement, error) {
 	if input == nil {
-		return nil, fmt.Errorf("create announcement: nil input")
+		return nil, ErrAnnouncementNilInput
 	}
 
 	title := strings.TrimSpace(input.Title)
 	content := strings.TrimSpace(input.Content)
 	if title == "" || len(title) > 200 {
-		return nil, fmt.Errorf("create announcement: invalid title")
+		return nil, ErrAnnouncementInvalidTitle
 	}
 	if content == "" {
-		return nil, fmt.Errorf("create announcement: content is required")
+		return nil, ErrAnnouncementContentRequired
 	}
 
 	status := strings.TrimSpace(input.Status)
@@ -87,7 +87,7 @@ func (s *AnnouncementService) Create(ctx context.Context, input *CreateAnnouncem
 		status = AnnouncementStatusDraft
 	}
 	if !isValidAnnouncementStatus(status) {
-		return nil, fmt.Errorf("create announcement: invalid status")
+		return nil, ErrAnnouncementInvalidStatus
 	}
 
 	targeting, err := domain.AnnouncementTargeting(input.Targeting).NormalizeAndValidate()
@@ -100,12 +100,12 @@ func (s *AnnouncementService) Create(ctx context.Context, input *CreateAnnouncem
 		notifyMode = AnnouncementNotifyModeSilent
 	}
 	if !isValidAnnouncementNotifyMode(notifyMode) {
-		return nil, fmt.Errorf("create announcement: invalid notify_mode")
+		return nil, ErrAnnouncementInvalidNotifyMode
 	}
 
 	if input.StartsAt != nil && input.EndsAt != nil {
 		if !input.StartsAt.Before(*input.EndsAt) {
-			return nil, fmt.Errorf("create announcement: starts_at must be before ends_at")
+			return nil, ErrAnnouncementInvalidSchedule
 		}
 	}
 
@@ -131,7 +131,7 @@ func (s *AnnouncementService) Create(ctx context.Context, input *CreateAnnouncem
 
 func (s *AnnouncementService) Update(ctx context.Context, id int64, input *UpdateAnnouncementInput) (*Announcement, error) {
 	if input == nil {
-		return nil, fmt.Errorf("update announcement: nil input")
+		return nil, ErrAnnouncementNilInput
 	}
 
 	a, err := s.announcementRepo.GetByID(ctx, id)
@@ -142,21 +142,21 @@ func (s *AnnouncementService) Update(ctx context.Context, id int64, input *Updat
 	if input.Title != nil {
 		title := strings.TrimSpace(*input.Title)
 		if title == "" || len(title) > 200 {
-			return nil, fmt.Errorf("update announcement: invalid title")
+			return nil, ErrAnnouncementInvalidTitle
 		}
 		a.Title = title
 	}
 	if input.Content != nil {
 		content := strings.TrimSpace(*input.Content)
 		if content == "" {
-			return nil, fmt.Errorf("update announcement: content is required")
+			return nil, ErrAnnouncementContentRequired
 		}
 		a.Content = content
 	}
 	if input.Status != nil {
 		status := strings.TrimSpace(*input.Status)
 		if !isValidAnnouncementStatus(status) {
-			return nil, fmt.Errorf("update announcement: invalid status")
+			return nil, ErrAnnouncementInvalidStatus
 		}
 		a.Status = status
 	}
@@ -164,7 +164,7 @@ func (s *AnnouncementService) Update(ctx context.Context, id int64, input *Updat
 	if input.NotifyMode != nil {
 		notifyMode := strings.TrimSpace(*input.NotifyMode)
 		if !isValidAnnouncementNotifyMode(notifyMode) {
-			return nil, fmt.Errorf("update announcement: invalid notify_mode")
+			return nil, ErrAnnouncementInvalidNotifyMode
 		}
 		a.NotifyMode = notifyMode
 	}
@@ -186,7 +186,7 @@ func (s *AnnouncementService) Update(ctx context.Context, id int64, input *Updat
 
 	if a.StartsAt != nil && a.EndsAt != nil {
 		if !a.StartsAt.Before(*a.EndsAt) {
-			return nil, fmt.Errorf("update announcement: starts_at must be before ends_at")
+			return nil, ErrAnnouncementInvalidSchedule
 		}
 	}
 
diff --git a/backend/internal/service/announcement_service_test.go b/backend/internal/service/announcement_service_test.go
new file mode 100644
index 00000000..77fb9896
--- /dev/null
+++ b/backend/internal/service/announcement_service_test.go
@@ -0,0 +1,81 @@
+package service
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
+	"github.com/stretchr/testify/require"
+)
+
+type announcementRepoStub struct {
+	item *Announcement
+}
+
+func (s *announcementRepoStub) Create(_ context.Context, a *Announcement) error {
+	s.item = a
+	return nil
+}
+
+func (s *announcementRepoStub) GetByID(_ context.Context, _ int64) (*Announcement, error) {
+	if s.item == nil {
+		return nil, ErrAnnouncementNotFound
+	}
+	return s.item, nil
+}
+
+func (s *announcementRepoStub) Update(_ context.Context, a *Announcement) error {
+	s.item = a
+	return nil
+}
+
+func (*announcementRepoStub) Delete(context.Context, int64) error {
+	return nil
+}
+
+func (*announcementRepoStub) List(context.Context, pagination.PaginationParams, AnnouncementListFilters) ([]Announcement, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+
+func (*announcementRepoStub) ListActive(context.Context, time.Time) ([]Announcement, error) {
+	return nil, nil
+}
+
+func TestAnnouncementServiceCreateRejectsEqualStartEndTimes(t *testing.T) {
+	repo := &announcementRepoStub{}
+	svc := NewAnnouncementService(repo, nil, nil, nil)
+	now := time.Unix(1776790020, 0)
+
+	_, err := svc.Create(context.Background(), &CreateAnnouncementInput{
+		Title:      "公告",
+		Content:    "内容",
+		Status:     AnnouncementStatusActive,
+		NotifyMode: AnnouncementNotifyModePopup,
+		StartsAt:   &now,
+		EndsAt:     &now,
+	})
+	require.ErrorIs(t, err, ErrAnnouncementInvalidSchedule)
+}
+
+func TestAnnouncementServiceUpdateRejectsEqualStartEndTimes(t *testing.T) {
+	repo := &announcementRepoStub{
+		item: &Announcement{
+			ID:         1,
+			Title:      "公告",
+			Content:    "内容",
+			Status:     AnnouncementStatusActive,
+			NotifyMode: AnnouncementNotifyModePopup,
+		},
+	}
+	svc := NewAnnouncementService(repo, nil, nil, nil)
+	now := time.Unix(1776790020, 0)
+	startsAt := &now
+	endsAt := &now
+
+	_, err := svc.Update(context.Background(), 1, &UpdateAnnouncementInput{
+		StartsAt: &startsAt,
+		EndsAt:   &endsAt,
+	})
+	require.ErrorIs(t, err, ErrAnnouncementInvalidSchedule)
+}