diff --git a/cmd/argocd/commands/app.go b/cmd/argocd/commands/app.go
index 00c5c14834e2f..60dbcc0d4418b 100644
--- a/cmd/argocd/commands/app.go
+++ b/cmd/argocd/commands/app.go
@@ -585,8 +585,8 @@ func printAppSummaryTable(app *argoappv1.Application, appURL string, windows *ar
 	var status string
 	var allow, deny, inactiveAllows bool
 	if windows.HasWindows() {
-		active := windows.Active()
-		if active.HasWindows() {
+		active, err := windows.Active()
+		if err == nil && active.HasWindows() {
 			for _, w := range *active {
 				if w.Kind == "deny" {
 					deny = true
@@ -595,13 +595,14 @@ func printAppSummaryTable(app *argoappv1.Application, appURL string, windows *ar
 				}
 			}
 		}
-		if windows.InactiveAllows().HasWindows() {
+		inactiveAllowWindows, err := windows.InactiveAllows()
+		if err == nil && inactiveAllowWindows.HasWindows() {
 			inactiveAllows = true
 		}
 
-		s := windows.CanSync(true)
 		if deny || !deny && !allow && inactiveAllows {
-			if s {
+			s, err := windows.CanSync(true)
+			if err == nil && s {
 				status = "Manual Allowed"
 			} else {
 				status = "Sync Denied"
diff --git a/cmd/argocd/commands/projectwindows.go b/cmd/argocd/commands/projectwindows.go
index d824222306419..b04615e22fd41 100644
--- a/cmd/argocd/commands/projectwindows.go
+++ b/cmd/argocd/commands/projectwindows.go
@@ -352,9 +352,10 @@ func printSyncWindows(proj *v1alpha1.AppProject) {
 	fmt.Fprintf(w, fmtStr, headers...)
 	if proj.Spec.SyncWindows.HasWindows() {
 		for i, window := range proj.Spec.SyncWindows {
+			isActive, _ := window.Active()
 			vals := []interface{}{
 				strconv.Itoa(i),
-				formatBoolOutput(window.Active()),
+				formatBoolOutput(isActive),
 				window.Kind,
 				window.Schedule,
 				window.Duration,
diff --git a/controller/appcontroller.go b/controller/appcontroller.go
index e64db6eae2ecc..e1cc6b3477846 100644
--- a/controller/appcontroller.go
+++ b/controller/appcontroller.go
@@ -1690,7 +1690,8 @@ func (ctrl *ApplicationController) processAppRefreshQueueItem() (processNext boo
 		app.Status.Summary = tree.GetSummary(app)
 	}
 
-	if project.Spec.SyncWindows.Matches(app).CanSync(false) {
+	canSync, _ := project.Spec.SyncWindows.Matches(app).CanSync(false)
+	if canSync {
 		syncErrCond, opMS := ctrl.autoSync(app, compareResult.syncStatus, compareResult.resources, compareResult.revisionUpdated)
 		setOpMs = opMS
 		if syncErrCond != nil {
diff --git a/controller/sync.go b/controller/sync.go
index 83a03d8a253d9..e20a8b49ee78f 100644
--- a/controller/sync.go
+++ b/controller/sync.go
@@ -174,12 +174,18 @@ func (m *appStateManager) SyncAppState(app *v1alpha1.Application, state *v1alpha
 		state.Phase = common.OperationError
 		state.Message = fmt.Sprintf("Failed to load application project: %v", err)
 		return
-	} else if syncWindowPreventsSync(app, proj) {
-		// If the operation is currently running, simply let the user know the sync is blocked by a current sync window
-		if state.Phase == common.OperationRunning {
-			state.Message = "Sync operation blocked by sync window"
+	} else {
+		isBlocked, err := syncWindowPreventsSync(app, proj)
+		if isBlocked {
+			// If the operation is currently running, simply let the user know the sync is blocked by a current sync window
+			if state.Phase == common.OperationRunning {
+				state.Message = "Sync operation blocked by sync window"
+				if err != nil {
+					state.Message = fmt.Sprintf("%s: %v", state.Message, err)
+				}
+			}
+			return
 		}
-		return
 	}
 
 	if !isMultiSourceRevision {
@@ -557,13 +563,18 @@ func delayBetweenSyncWaves(phase common.SyncPhase, wave int, finalWave bool) err
 	return nil
 }
 
-func syncWindowPreventsSync(app *v1alpha1.Application, proj *v1alpha1.AppProject) bool {
+func syncWindowPreventsSync(app *v1alpha1.Application, proj *v1alpha1.AppProject) (bool, error) {
 	window := proj.Spec.SyncWindows.Matches(app)
 	isManual := false
 	if app.Status.OperationState != nil {
 		isManual = !app.Status.OperationState.Operation.InitiatedBy.Automated
 	}
-	return !window.CanSync(isManual)
+	canSync, err := window.CanSync(isManual)
+	if err != nil {
+		// prevents sync because sync window has an error
+		return true, err
+	}
+	return !canSync, nil
 }
 
 // deriveServiceAccountToImpersonate determines the service account to be used for impersonation for the sync operation.
diff --git a/pkg/apis/application/v1alpha1/types.go b/pkg/apis/application/v1alpha1/types.go
index 40515b220832a..697c98f19fcf1 100644
--- a/pkg/apis/application/v1alpha1/types.go
+++ b/pkg/apis/application/v1alpha1/types.go
@@ -2379,11 +2379,11 @@ func (s *SyncWindows) HasWindows() bool {
 }
 
 // Active returns a list of sync windows that are currently active
-func (s *SyncWindows) Active() *SyncWindows {
+func (s *SyncWindows) Active() (*SyncWindows, error) {
 	return s.active(time.Now())
 }
 
-func (s *SyncWindows) active(currentTime time.Time) *SyncWindows {
+func (s *SyncWindows) active(currentTime time.Time) (*SyncWindows, error) {
 	// If SyncWindows.Active() is called outside of a UTC locale, it should be
 	// first converted to UTC before we scan through the SyncWindows.
 	currentTime = currentTime.In(time.UTC)
@@ -2392,8 +2392,14 @@ func (s *SyncWindows) active(currentTime time.Time) *SyncWindows {
 		var active SyncWindows
 		specParser := cron.NewParser(cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow)
 		for _, w := range *s {
-			schedule, _ := specParser.Parse(w.Schedule)
-			duration, _ := time.ParseDuration(w.Duration)
+			schedule, sErr := specParser.Parse(w.Schedule)
+			if sErr != nil {
+				return nil, fmt.Errorf("cannot parse schedule '%s': %w", w.Schedule, sErr)
+			}
+			duration, dErr := time.ParseDuration(w.Duration)
+			if dErr != nil {
+				return nil, fmt.Errorf("cannot parse duration '%s': %w", w.Duration, dErr)
+			}
 
 			// Offset the nextWindow time to consider the timeZone of the sync window
 			timeZoneOffsetDuration := w.scheduleOffsetByTimeZone()
@@ -2403,20 +2409,20 @@ func (s *SyncWindows) active(currentTime time.Time) *SyncWindows {
 			}
 		}
 		if len(active) > 0 {
-			return &active
+			return &active, nil
 		}
 	}
-	return nil
+	return nil, nil
 }
 
 // InactiveAllows will iterate over the SyncWindows and return all inactive allow windows
 // for the current time. If the current time is in an inactive allow window, syncs will
 // be denied.
-func (s *SyncWindows) InactiveAllows() *SyncWindows {
+func (s *SyncWindows) InactiveAllows() (*SyncWindows, error) {
 	return s.inactiveAllows(time.Now())
 }
 
-func (s *SyncWindows) inactiveAllows(currentTime time.Time) *SyncWindows {
+func (s *SyncWindows) inactiveAllows(currentTime time.Time) (*SyncWindows, error) {
 	// If SyncWindows.InactiveAllows() is called outside of a UTC locale, it should be
 	// first converted to UTC before we scan through the SyncWindows.
 	currentTime = currentTime.In(time.UTC)
@@ -2427,21 +2433,27 @@ func (s *SyncWindows) inactiveAllows(currentTime time.Time) *SyncWindows {
 		for _, w := range *s {
 			if w.Kind == "allow" {
 				schedule, sErr := specParser.Parse(w.Schedule)
+				if sErr != nil {
+					return nil, fmt.Errorf("cannot parse schedule '%s': %w", w.Schedule, sErr)
+				}
 				duration, dErr := time.ParseDuration(w.Duration)
+				if dErr != nil {
+					return nil, fmt.Errorf("cannot parse duration '%s': %w", w.Duration, dErr)
+				}
 				// Offset the nextWindow time to consider the timeZone of the sync window
 				timeZoneOffsetDuration := w.scheduleOffsetByTimeZone()
 				nextWindow := schedule.Next(currentTime.Add(timeZoneOffsetDuration - duration))
 
-				if !nextWindow.Before(currentTime.Add(timeZoneOffsetDuration)) && sErr == nil && dErr == nil {
+				if !nextWindow.Before(currentTime.Add(timeZoneOffsetDuration)) {
 					inactive = append(inactive, w)
 				}
 			}
 		}
 		if len(inactive) > 0 {
-			return &inactive
+			return &inactive, nil
 		}
 	}
-	return nil
+	return nil, nil
 }
 
 func (w *SyncWindow) scheduleOffsetByTimeZone() time.Duration {
@@ -2545,36 +2557,42 @@ func (w *SyncWindows) Matches(app *Application) *SyncWindows {
 }
 
 // CanSync returns true if a sync window currently allows a sync. isManual indicates whether the sync has been triggered manually.
-func (w *SyncWindows) CanSync(isManual bool) bool {
+func (w *SyncWindows) CanSync(isManual bool) (bool, error) {
 	if !w.HasWindows() {
-		return true
+		return true, nil
 	}
 
-	active := w.Active()
+	active, err := w.Active()
+	if err != nil {
+		return false, fmt.Errorf("invalid sync windows: %w", err)
+	}
 	hasActiveDeny, manualEnabled := active.hasDeny()
 
 	if hasActiveDeny {
 		if isManual && manualEnabled {
-			return true
+			return true, nil
 		} else {
-			return false
+			return false, nil
 		}
 	}
 
 	if active.hasAllow() {
-		return true
+		return true, nil
 	}
 
-	inactiveAllows := w.InactiveAllows()
+	inactiveAllows, err := w.InactiveAllows()
+	if err != nil {
+		return false, fmt.Errorf("invalid sync windows: %w", err)
+	}
 	if inactiveAllows.HasWindows() {
 		if isManual && inactiveAllows.manualEnabled() {
-			return true
+			return true, nil
 		} else {
-			return false
+			return false, nil
 		}
 	}
 
-	return true
+	return true, nil
 }
 
 // hasDeny will iterate over the SyncWindows and return if a deny window is found and if
@@ -2629,24 +2647,30 @@ func (w *SyncWindows) manualEnabled() bool {
 }
 
 // Active returns true if the sync window is currently active
-func (w SyncWindow) Active() bool {
+func (w SyncWindow) Active() (bool, error) {
 	return w.active(time.Now())
 }
 
-func (w SyncWindow) active(currentTime time.Time) bool {
+func (w SyncWindow) active(currentTime time.Time) (bool, error) {
 	// If SyncWindow.Active() is called outside of a UTC locale, it should be
 	// first converted to UTC before search
 	currentTime = currentTime.UTC()
 
 	specParser := cron.NewParser(cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow)
-	schedule, _ := specParser.Parse(w.Schedule)
-	duration, _ := time.ParseDuration(w.Duration)
+	schedule, sErr := specParser.Parse(w.Schedule)
+	if sErr != nil {
+		return false, fmt.Errorf("cannot parse schedule '%s': %w", w.Schedule, sErr)
+	}
+	duration, dErr := time.ParseDuration(w.Duration)
+	if dErr != nil {
+		return false, fmt.Errorf("cannot parse duration '%s': %w", w.Duration, dErr)
+	}
 
 	// Offset the nextWindow time to consider the timeZone of the sync window
 	timeZoneOffsetDuration := w.scheduleOffsetByTimeZone()
 	nextWindow := schedule.Next(currentTime.Add(timeZoneOffsetDuration - duration))
 
-	return nextWindow.Before(currentTime.Add(timeZoneOffsetDuration))
+	return nextWindow.Before(currentTime.Add(timeZoneOffsetDuration)), nil
 }
 
 // Update updates a sync window's settings with the given parameter
diff --git a/pkg/apis/application/v1alpha1/types_test.go b/pkg/apis/application/v1alpha1/types_test.go
index e3ab6e3ef650c..056760fb0d1e7 100644
--- a/pkg/apis/application/v1alpha1/types_test.go
+++ b/pkg/apis/application/v1alpha1/types_test.go
@@ -1778,7 +1778,9 @@ func TestSyncWindows_HasWindows(t *testing.T) {
 func TestSyncWindows_Active(t *testing.T) {
 	t.Run("WithTestProject", func(t *testing.T) {
 		proj := newTestProjectWithSyncWindows()
-		assert.Len(t, *proj.Spec.SyncWindows.Active(), 1)
+		activeWindows, err := proj.Spec.SyncWindows.Active()
+		require.NoError(t, err)
+		assert.Len(t, *activeWindows, 1)
 	})
 
 	syncWindow := func(kind string, schedule string, duration string, timeZone string) *SyncWindow {
@@ -1805,6 +1807,7 @@ func TestSyncWindows_Active(t *testing.T) {
 		currentTime    time.Time
 		matchingIndex  int
 		expectedLength int
+		isErr          bool
 	}{
 		{
 			name: "MatchFirst",
@@ -1912,11 +1915,36 @@ func TestSyncWindows_Active(t *testing.T) {
 			matchingIndex:  0,
 			expectedLength: 1,
 		},
+		{
+			name: "MatchNone-InvalidSchedule",
+			syncWindow: SyncWindows{
+				syncWindow("allow", "* 10 * * 7", "3h", ""),
+				syncWindow("allow", "* 11 * * 7", "3h", ""),
+			},
+			currentTime:    timeWithHour(12, time.UTC),
+			expectedLength: 0,
+			isErr:          true,
+		},
+		{
+			name: "MatchNone-InvalidDuration",
+			syncWindow: SyncWindows{
+				syncWindow("allow", "* 10 * * *", "3a", ""),
+				syncWindow("allow", "* 11 * * *", "3a", ""),
+			},
+			currentTime:    timeWithHour(12, time.UTC),
+			expectedLength: 0,
+			isErr:          true,
+		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			result := tt.syncWindow.active(tt.currentTime)
+			result, err := tt.syncWindow.active(tt.currentTime)
+			if tt.isErr {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+			}
 			if result == nil {
 				result = &SyncWindows{}
 			}
@@ -1933,7 +1961,9 @@ func TestSyncWindows_InactiveAllows(t *testing.T) {
 	t.Run("WithTestProject", func(t *testing.T) {
 		proj := newTestProjectWithSyncWindows()
 		proj.Spec.SyncWindows[0].Schedule = "0 0 1 1 1"
-		assert.Len(t, *proj.Spec.SyncWindows.InactiveAllows(), 1)
+		inactiveAllowWindows, err := proj.Spec.SyncWindows.InactiveAllows()
+		require.NoError(t, err)
+		assert.Len(t, *inactiveAllowWindows, 1)
 	})
 
 	syncWindow := func(kind string, schedule string, duration string, timeZone string) *SyncWindow {
@@ -1960,6 +1990,7 @@ func TestSyncWindows_InactiveAllows(t *testing.T) {
 		currentTime    time.Time
 		matchingIndex  int
 		expectedLength int
+		isErr          bool
 	}{
 		{
 			name: "MatchFirst",
@@ -2085,11 +2116,34 @@ func TestSyncWindows_InactiveAllows(t *testing.T) {
 			matchingIndex:  0,
 			expectedLength: 1,
 		},
+		{
+			name: "MatchNone-InvalidSchedule",
+			syncWindow: SyncWindows{
+				syncWindow("allow", "* 10 * * 7", "2h", ""),
+			},
+			currentTime:    timeWithHour(17, time.UTC),
+			expectedLength: 0,
+			isErr:          true,
+		},
+		{
+			name: "MatchNone-InvalidDuration",
+			syncWindow: SyncWindows{
+				syncWindow("allow", "* 10 * * *", "2a", ""),
+			},
+			currentTime:    timeWithHour(17, time.UTC),
+			expectedLength: 0,
+			isErr:          true,
+		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			result := tt.syncWindow.inactiveAllows(tt.currentTime)
+			result, err := tt.syncWindow.inactiveAllows(tt.currentTime)
+			if tt.isErr {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+			}
 			if result == nil {
 				result = &SyncWindows{}
 			}
@@ -2200,9 +2254,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 		proj := newProjectBuilder().withInactiveDenyWindow(true).build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will allow manual sync if inactive-deny-window set with manual false", func(t *testing.T) {
@@ -2211,9 +2266,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 		proj := newProjectBuilder().withInactiveDenyWindow(false).build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will deny manual sync if one inactive-allow-windows set with manual false", func(t *testing.T) {
@@ -2225,9 +2281,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will allow manual sync if on active-allow-window set with manual true", func(t *testing.T) {
@@ -2238,9 +2295,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will allow manual sync if on active-allow-window set with manual false", func(t *testing.T) {
@@ -2251,9 +2309,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will allow auto sync if on active-allow-window", func(t *testing.T) {
@@ -2264,9 +2323,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will allow manual sync active-allow and inactive-deny", func(t *testing.T) {
@@ -2278,9 +2338,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will allow auto sync active-allow and inactive-deny", func(t *testing.T) {
@@ -2292,9 +2353,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will deny manual sync inactive-allow", func(t *testing.T) {
@@ -2305,9 +2367,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will deny auto sync inactive-allow", func(t *testing.T) {
@@ -2318,9 +2381,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will allow manual sync inactive-allow with ManualSync enabled", func(t *testing.T) {
@@ -2331,9 +2395,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will deny auto sync inactive-allow with ManualSync enabled", func(t *testing.T) {
@@ -2344,9 +2409,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will deny manual sync with inactive-allow and inactive-deny", func(t *testing.T) {
@@ -2358,9 +2424,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will deny auto sync with inactive-allow and inactive-deny", func(t *testing.T) {
@@ -2372,9 +2439,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will allow auto sync with active-allow and inactive-allow", func(t *testing.T) {
@@ -2386,9 +2454,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will deny manual sync with active-deny", func(t *testing.T) {
@@ -2399,9 +2468,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will deny auto sync with active-deny", func(t *testing.T) {
@@ -2412,9 +2482,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will allow manual sync with active-deny with ManualSync enabled", func(t *testing.T) {
@@ -2425,9 +2496,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will deny auto sync with active-deny with ManualSync enabled", func(t *testing.T) {
@@ -2438,9 +2510,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will deny manual sync with many active-deny having one with ManualSync disabled", func(t *testing.T) {
@@ -2454,9 +2527,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will deny auto sync with many active-deny having one with ManualSync disabled", func(t *testing.T) {
@@ -2470,9 +2544,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will deny manual sync with active-deny and active-allow windows with ManualSync disabled", func(t *testing.T) {
@@ -2484,9 +2559,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.False(t, canSync)
 	})
 	t.Run("will allow manual sync with active-deny and active-allow windows with ManualSync enabled", func(t *testing.T) {
@@ -2498,9 +2574,10 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(true)
+		canSync, err := proj.Spec.SyncWindows.CanSync(true)
 
 		// then
+		require.NoError(t, err)
 		assert.True(t, canSync)
 	})
 	t.Run("will deny auto sync with active-deny and active-allow windows with ManualSync enabled", func(t *testing.T) {
@@ -2512,9 +2589,24 @@ func TestSyncWindows_CanSync(t *testing.T) {
 			build()
 
 		// when
-		canSync := proj.Spec.SyncWindows.CanSync(false)
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
+
+		// then
+		require.NoError(t, err)
+		assert.False(t, canSync)
+	})
+	t.Run("will deny and return error with invalid windows", func(t *testing.T) {
+		// given
+		t.Parallel()
+		proj := newProjectBuilder().
+			withInvalidWindows().
+			build()
+
+		// when
+		canSync, err := proj.Spec.SyncWindows.CanSync(false)
 
 		// then
+		require.Error(t, err)
 		assert.False(t, canSync)
 	})
 }
@@ -2564,8 +2656,9 @@ func TestSyncWindows_hasAllow(t *testing.T) {
 func TestSyncWindow_Active(t *testing.T) {
 	window := &SyncWindow{Schedule: "* * * * *", Duration: "1h"}
 	t.Run("ActiveWindow", func(t *testing.T) {
-		window.Active()
-		assert.True(t, window.Active())
+		isActive, err := window.Active()
+		require.NoError(t, err)
+		assert.True(t, isActive)
 	})
 
 	syncWindow := func(kind string, schedule string, duration string) SyncWindow {
@@ -2590,6 +2683,7 @@ func TestSyncWindow_Active(t *testing.T) {
 		syncWindow     SyncWindow
 		currentTime    time.Time
 		expectedResult bool
+		isErr          bool
 	}{
 		{
 			name:           "Allow-active",
@@ -2639,11 +2733,44 @@ func TestSyncWindow_Active(t *testing.T) {
 			currentTime:    timeWithHour(13-4, utcM4Zone),
 			expectedResult: false,
 		},
+		{
+			name:           "Allow-inactive-InvalidSchedule",
+			syncWindow:     syncWindow("allow", "* 10 * * 7", "2h"),
+			currentTime:    timeWithHour(11, time.UTC),
+			expectedResult: false,
+			isErr:          true,
+		},
+		{
+			name:           "Deny-inactive-InvalidSchedule",
+			syncWindow:     syncWindow("deny", "* 10 * * 7", "2h"),
+			currentTime:    timeWithHour(11, time.UTC),
+			expectedResult: false,
+			isErr:          true,
+		},
+		{
+			name:           "Allow-inactive-InvalidDuration",
+			syncWindow:     syncWindow("allow", "* 10 * * *", "2a"),
+			currentTime:    timeWithHour(11, time.UTC),
+			expectedResult: false,
+			isErr:          true,
+		},
+		{
+			name:           "Deny-inactive-InvalidDuration",
+			syncWindow:     syncWindow("deny", "* 10 * * *", "2a"),
+			currentTime:    timeWithHour(11, time.UTC),
+			expectedResult: false,
+			isErr:          true,
+		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			result := tt.syncWindow.active(tt.currentTime)
+			result, err := tt.syncWindow.active(tt.currentTime)
+			if tt.isErr {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+			}
 			assert.Equal(t, tt.expectedResult, result)
 		})
 	}
@@ -2755,6 +2882,16 @@ func (b *projectBuilder) withInactiveDenyWindow(allowManual bool) *projectBuilde
 	return b
 }
 
+func (b *projectBuilder) withInvalidWindows() *projectBuilder {
+	b.proj.Spec.SyncWindows = append(b.proj.Spec.SyncWindows,
+		newSyncWindow("allow", "* 10 * * 7", false),
+		newSyncWindow("deny", "* 10 * * 7", false),
+		newSyncWindow("allow", "* 10 * * 7", true),
+		newSyncWindow("deny", "* 10 * * 7", true),
+	)
+	return b
+}
+
 func inactiveCronSchedule() string {
 	hourPlus10, _, _ := time.Now().Add(10 * time.Hour).Clock()
 	return fmt.Sprintf("0 %d * * *", hourPlus10)
diff --git a/server/application/application.go b/server/application/application.go
index f5eecd9d8c6ad..0f6542e35de32 100644
--- a/server/application/application.go
+++ b/server/application/application.go
@@ -1886,7 +1886,11 @@ func (s *Server) Sync(ctx context.Context, syncReq *application.ApplicationSyncR
 
 	s.inferResourcesStatusHealth(a)
 
-	if !proj.Spec.SyncWindows.Matches(a).CanSync(true) {
+	canSync, err := proj.Spec.SyncWindows.Matches(a).CanSync(true)
+	if err != nil {
+		return a, status.Errorf(codes.PermissionDenied, "cannot sync: invalid sync window: %v", err)
+	}
+	if !canSync {
 		return a, status.Errorf(codes.PermissionDenied, "cannot sync: blocked by sync window")
 	}
 
@@ -2603,10 +2607,17 @@ func (s *Server) GetApplicationSyncWindows(ctx context.Context, q *application.A
 	}
 
 	windows := proj.Spec.SyncWindows.Matches(a)
-	sync := windows.CanSync(true)
+	sync, err := windows.CanSync(true)
+	if err != nil {
+		return nil, fmt.Errorf("invalid sync windows: %w", err)
+	}
 
+	activeWindows, err := windows.Active()
+	if err != nil {
+		return nil, fmt.Errorf("invalid sync windows: %w", err)
+	}
 	res := &application.ApplicationSyncWindowsResponse{
-		ActiveWindows:   convertSyncWindows(windows.Active()),
+		ActiveWindows:   convertSyncWindows(activeWindows),
 		AssignedWindows: convertSyncWindows(windows),
 		CanSync:         &sync,
 	}
diff --git a/server/project/project.go b/server/project/project.go
index 3406c87315259..83722bb20ec0e 100644
--- a/server/project/project.go
+++ b/server/project/project.go
@@ -525,7 +525,10 @@ func (s *Server) GetSyncWindowsState(ctx context.Context, q *project.SyncWindows
 
 	res := &project.SyncWindowsResponse{}
 
-	windows := proj.Spec.SyncWindows.Active()
+	windows, err := proj.Spec.SyncWindows.Active()
+	if err != nil {
+		return nil, err
+	}
 	if windows.HasWindows() {
 		res.Windows = *windows
 	} else {