diff --git a/model/alert.go b/model/alert.go
index 35e739c7..a0c7a667 100644
--- a/model/alert.go
+++ b/model/alert.go
@@ -34,6 +34,8 @@ type Alert struct {
 	// Extra key/value information which does not define alert identity.
 	Annotations LabelSet `json:"annotations"`
 
+	ActiveAt time.Time `json:"activeAt,omitempty"`
+
 	// The known time range for this alert. Both ends are optional.
 	StartsAt     time.Time `json:"startsAt,omitempty"`
 	EndsAt       time.Time `json:"endsAt,omitempty"`
@@ -83,9 +85,15 @@ func (a *Alert) Status() AlertStatus {
 
 // Validate checks whether the alert data is inconsistent.
 func (a *Alert) Validate() error {
+	if a.ActiveAt.IsZero() {
+		return fmt.Errorf("active time missing")
+	}
 	if a.StartsAt.IsZero() {
 		return fmt.Errorf("start time missing")
 	}
+	if a.StartsAt.Before(a.ActiveAt) {
+		return fmt.Errorf("active time must be before start time")
+	}
 	if !a.EndsAt.IsZero() && a.EndsAt.Before(a.StartsAt) {
 		return fmt.Errorf("start time must be before end time")
 	}
diff --git a/model/alert_test.go b/model/alert_test.go
index 8910da7a..0de45717 100644
--- a/model/alert_test.go
+++ b/model/alert_test.go
@@ -23,7 +23,6 @@ import (
 
 func TestAlertValidate(t *testing.T) {
 	ts := time.Now()
-
 	var cases = []struct {
 		alert *Alert
 		err   string
@@ -31,18 +30,27 @@ func TestAlertValidate(t *testing.T) {
 		{
 			alert: &Alert{
 				Labels:   LabelSet{"a": "b"},
-				StartsAt: ts,
+				ActiveAt: ts,
+				StartsAt: ts.Add(time.Second),
 			},
 		},
 		{
 			alert: &Alert{
 				Labels: LabelSet{"a": "b"},
 			},
+			err: "active time missing",
+		},
+		{
+			alert: &Alert{
+				Labels:   LabelSet{"a": "b"},
+				ActiveAt: ts,
+			},
 			err: "start time missing",
 		},
 		{
 			alert: &Alert{
 				Labels:   LabelSet{"a": "b"},
+				ActiveAt: ts,
 				StartsAt: ts,
 				EndsAt:   ts,
 			},
@@ -50,6 +58,15 @@ func TestAlertValidate(t *testing.T) {
 		{
 			alert: &Alert{
 				Labels:   LabelSet{"a": "b"},
+				ActiveAt: ts,
+				StartsAt: ts.Add(time.Second),
+				EndsAt:   ts.Add(1 * time.Minute),
+			},
+		},
+		{
+			alert: &Alert{
+				Labels:   LabelSet{"a": "b"},
+				ActiveAt: ts,
 				StartsAt: ts,
 				EndsAt:   ts.Add(1 * time.Minute),
 			},
@@ -57,6 +74,16 @@ func TestAlertValidate(t *testing.T) {
 		{
 			alert: &Alert{
 				Labels:   LabelSet{"a": "b"},
+				ActiveAt: ts,
+				StartsAt: ts.Add(-1 * time.Minute),
+				EndsAt:   ts.Add(-1 * time.Minute),
+			},
+			err: "active time must be before start time",
+		},
+		{
+			alert: &Alert{
+				Labels:   LabelSet{"a": "b"},
+				ActiveAt: ts,
 				StartsAt: ts,
 				EndsAt:   ts.Add(-1 * time.Minute),
 			},
@@ -64,6 +91,7 @@ func TestAlertValidate(t *testing.T) {
 		},
 		{
 			alert: &Alert{
+				ActiveAt: ts,
 				StartsAt: ts,
 			},
 			err: "at least one label pair required",
@@ -71,6 +99,7 @@ func TestAlertValidate(t *testing.T) {
 		{
 			alert: &Alert{
 				Labels:   LabelSet{"a": "b", "!bad": "label"},
+				ActiveAt: ts,
 				StartsAt: ts,
 			},
 			err: "invalid label set: invalid name",
@@ -78,6 +107,7 @@ func TestAlertValidate(t *testing.T) {
 		{
 			alert: &Alert{
 				Labels:   LabelSet{"a": "b", "bad": "\xfflabel"},
+				ActiveAt: ts,
 				StartsAt: ts,
 			},
 			err: "invalid label set: invalid value",
@@ -86,6 +116,7 @@ func TestAlertValidate(t *testing.T) {
 			alert: &Alert{
 				Labels:      LabelSet{"a": "b"},
 				Annotations: LabelSet{"!bad": "label"},
+				ActiveAt:    ts,
 				StartsAt:    ts,
 			},
 			err: "invalid annotations: invalid name",
@@ -94,6 +125,7 @@ func TestAlertValidate(t *testing.T) {
 			alert: &Alert{
 				Labels:      LabelSet{"a": "b"},
 				Annotations: LabelSet{"bad": "\xfflabel"},
+				ActiveAt:    ts,
 				StartsAt:    ts,
 			},
 			err: "invalid annotations: invalid value",