diff --git a/backend/internal/repository/redeem_reduction_remainder_integration_test.go b/backend/internal/repository/redeem_reduction_remainder_integration_test.go
new file mode 100644
index 0000000000..0bb110807a
--- /dev/null
+++ b/backend/internal/repository/redeem_reduction_remainder_integration_test.go
@@ -0,0 +1,51 @@
+//go:build integration
+
+package repository
+
+import (
+	"context"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/stretchr/testify/require"
+	"testing"
+	"time"
+)
+
+func TestRedeemReductionPreservesPartialDay(t *testing.T) {
+	ctx := context.Background()
+	client := testEntClient(t)
+	user, err := client.User.Create().SetEmail("reduction-remainder@example.com").SetPasswordHash("test").Save(ctx)
+	require.NoError(t, err)
+	group, err := client.Group.Create().SetName("reduction-remainder").Save(ctx)
+	require.NoError(t, err)
+	// Redemption commits its own transaction; remove fixtures explicitly so
+	// repository list/count tests cannot observe this test's subscriptions.
+	t.Cleanup(func() {
+		_, err := integrationDB.ExecContext(context.Background(), `DELETE FROM redeem_codes WHERE group_id=$1`, group.ID)
+		require.NoError(t, err)
+		_, err = integrationDB.ExecContext(context.Background(), `DELETE FROM user_subscriptions WHERE user_id=$1`, user.ID)
+		require.NoError(t, err)
+		_, err = integrationDB.ExecContext(context.Background(), `DELETE FROM groups WHERE id=$1`, group.ID)
+		require.NoError(t, err)
+		_, err = integrationDB.ExecContext(context.Background(), `DELETE FROM users WHERE id=$1`, user.ID)
+		require.NoError(t, err)
+	})
+	now := time.Now().UTC().Truncate(time.Microsecond)
+	expiry := now.Add(36 * time.Hour)
+	sub, err := client.UserSubscription.Create().SetUserID(user.ID).SetGroupID(group.ID).SetStartsAt(now.Add(-time.Hour)).SetExpiresAt(expiry).SetStatus(service.SubscriptionStatusActive).Save(ctx)
+	require.NoError(t, err)
+	repo := NewUserSubscriptionRepository(client)
+	codes := NewRedeemCodeRepository(client)
+	code := &service.RedeemCode{Code: "REDUCE-ONE-DAY", Type: service.RedeemTypeSubscription, Status: service.StatusUnused, ValidityDays: -1, GroupID: &group.ID}
+	require.NoError(t, codes.Create(ctx, code))
+	subscriptions := service.NewSubscriptionService(nil, repo, nil, client, nil)
+	redeem := service.NewRedeemService(codes, NewUserRepository(client, integrationDB), subscriptions, nil, nil, client, nil, nil)
+	_, err = redeem.RedeemForAdminFulfillment(ctx, user.ID, code.Code)
+	require.NoError(t, err)
+	got, err := repo.GetByID(ctx, sub.ID)
+	require.NoError(t, err)
+	require.Equal(t, service.SubscriptionStatusActive, got.Status)
+	require.True(t, expiry.AddDate(0, 0, -1).Equal(got.ExpiresAt))
+	used, err := codes.GetByID(ctx, code.ID)
+	require.NoError(t, err)
+	require.Equal(t, service.StatusUsed, used.Status)
+}
diff --git a/backend/internal/service/redeem_service.go b/backend/internal/service/redeem_service.go
index 30dd4f911f..4894355add 100644
--- a/backend/internal/service/redeem_service.go
+++ b/backend/internal/service/redeem_service.go
@@ -711,14 +711,15 @@ func (s *RedeemService) reduceOrCancelSubscription(ctx context.Context, userID,
 	}
 
 	now := time.Now()
-	remaining := int(sub.ExpiresAt.Sub(now).Hours() / 24)
-	if remaining < 0 {
-		remaining = 0
+	if s.subscriptionService.now != nil {
+		now = s.subscriptionService.now()
 	}
+	// Preserve calendar-day semantics without rounding away the remaining hours.
+	newExpiresAt := sub.ExpiresAt.AddDate(0, 0, -reduceDays)
 
 	notes := fmt.Sprintf("通过兑换码 %s 退款扣减 %d 天", code, reduceDays)
 
-	if remaining <= reduceDays {
+	if !newExpiresAt.After(now) {
 		// 剩余天数不足，直接取消订阅
 		if err := s.subscriptionService.userSubRepo.UpdateStatus(ctx, sub.ID, SubscriptionStatusExpired); err != nil {
 			return fmt.Errorf("cancel subscription: %w", err)
@@ -729,7 +730,6 @@ func (s *RedeemService) reduceOrCancelSubscription(ctx context.Context, userID,
 		}
 	} else {
 		// 缩短天数
-		newExpiresAt := sub.ExpiresAt.AddDate(0, 0, -reduceDays)
 		if err := s.subscriptionService.userSubRepo.ExtendExpiry(ctx, sub.ID, newExpiresAt); err != nil {
 			return fmt.Errorf("reduce subscription: %w", err)
 		}
diff --git a/backend/internal/service/redeem_subscription_reduction_remainder_test.go b/backend/internal/service/redeem_subscription_reduction_remainder_test.go
new file mode 100644
index 0000000000..0e59f02fb6
--- /dev/null
+++ b/backend/internal/service/redeem_subscription_reduction_remainder_test.go
@@ -0,0 +1,44 @@
+package service
+
+import (
+	"context"
+	"github.com/stretchr/testify/require"
+	"testing"
+	"time"
+)
+
+func TestRedeemReductionPreservesRemainingTime(t *testing.T) {
+	now := time.Date(2026, 9, 22, 12, 0, 0, 0, time.UTC)
+	for _, tc := range []struct {
+		name      string
+		remaining time.Duration
+		days      int
+		expired   bool
+	}{
+		{"partial day", 36 * time.Hour, 1, false},
+		{"one second left", 24*time.Hour + time.Second, 1, false},
+		{"exact exhaustion", 24 * time.Hour, 1, true},
+		{"excess deduction", 12 * time.Hour, 1, true},
+		{"already expired", -time.Hour, 1, true},
+		{"multiple days", 84 * time.Hour, 2, false},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			sub := UserSubscription{ID: 7, ExpiresAt: now.Add(tc.remaining), Status: SubscriptionStatusActive, Notes: "original"}
+			repo := &lockingRenewalRepo{stale: sub, current: sub}
+			subscriptions := NewSubscriptionService(nil, repo, nil, nil, nil)
+			subscriptions.now = func() time.Time { return now }
+			svc := &RedeemService{subscriptionService: subscriptions}
+			require.NoError(t, svc.reduceOrCancelSubscription(context.Background(), 11, 13, tc.days, "deduct"))
+			want := sub.ExpiresAt.AddDate(0, 0, -tc.days)
+			status := SubscriptionStatusActive
+			if tc.expired {
+				want = now
+				status = SubscriptionStatusExpired
+			}
+			require.Equal(t, want, repo.current.ExpiresAt)
+			require.Equal(t, status, repo.current.Status)
+			require.Contains(t, repo.current.Notes, "original")
+			require.Contains(t, repo.current.Notes, "deduct")
+		})
+	}
+}