diff --git a/docs/spec/v1beta3/providers.md b/docs/spec/v1beta3/providers.md
index 080b82e43..ef59ce7e3 100644
--- a/docs/spec/v1beta3/providers.md
+++ b/docs/spec/v1beta3/providers.md
@@ -878,9 +878,10 @@ an [Event](events.md#event-structure) to the provided Prometheus Alertmanager
 [Address](#address).
 
 The Event will be formatted into a `firing` [Prometheus Alertmanager
-alert](https://prometheus.io/docs/alerting/latest/notifications/#alert),
-with the metadata added to the `labels` fields, and the `message` (and optional
-`.metadata.summary`) added as annotations.
+alert](https://prometheus.io/docs/alerting/latest/notifications/#alert), with
+the metadata added to the `labels` fields, and the `message` (and optional
+`.metadata.summary`) added as annotations. Event timestamp will be used to set
+alert start time (`.StartsAt`).
 
 In addition to the metadata from the Event, the following labels will be added:
 
@@ -888,12 +889,26 @@ In addition to the metadata from the Event, the following labels will be added:
 |-----------|------------------------------------------------------------------------------------------------------|
 | alertname | The string Flux followed by the Kind and the reason for the event e.g `FluxKustomizationProgressing` |
 | severity  | The severity of the event (`error` or `info`)                                                        |
-| timestamp | The timestamp of the event                                                                           |
 | reason    | The machine readable reason for the objects transition into the current status                       |
 | kind      | The kind of the involved object associated with the event                                            |
 | name      | The name of the involved object associated with the event                                            |
 | namespace | The namespace of the involved object associated with the event                                       |
 
+Note that due to the way other Flux controllers currently emit events, there's
+no way for notification-controller to figure out the time the event ends to set
+`.EndsAt` (a reasonable estimate being double the reconciliation interval of the
+resource involved) that doesn't involve a Kubernetes API roundtrip. A
+possible workaround could be setting
+[`global.resolve_timeout`][am_config_global] to an interval large enough for
+events to reoccur:
+
+[am_config_global]: https://prometheus.io/docs/alerting/latest/configuration/#file-layout-and-global-settings
+
+```yaml
+global:
+  resolve_timeout: 1h
+```
+
 This Provider type does support the configuration of a [proxy URL](#https-proxy)
 and [TLS certificates](#tls-certificates).
 
diff --git a/internal/notifier/alertmanager.go b/internal/notifier/alertmanager.go
index 3bfa352d0..b1cb741bb 100644
--- a/internal/notifier/alertmanager.go
+++ b/internal/notifier/alertmanager.go
@@ -19,8 +19,10 @@ package notifier
 import (
 	"context"
 	"crypto/x509"
+	"encoding/json"
 	"fmt"
 	"net/url"
+	"time"
 
 	"golang.org/x/text/cases"
 	"golang.org/x/text/language"
@@ -38,6 +40,36 @@ type AlertManagerAlert struct {
 	Status      string            `json:"status"`
 	Labels      map[string]string `json:"labels"`
 	Annotations map[string]string `json:"annotations"`
+
+	StartsAt AlertManagerTime `json:"startsAt"`
+	EndsAt   AlertManagerTime `json:"endsAt,omitempty"`
+}
+
+// AlertManagerTime takes care of representing time.Time as RFC3339.
+// See https://prometheus.io/docs/alerting/0.27/clients/
+type AlertManagerTime time.Time
+
+func (a AlertManagerTime) String() string {
+	return time.Time(a).Format(time.RFC3339)
+}
+
+func (a AlertManagerTime) MarshalJSON() ([]byte, error) {
+	return json.Marshal(a.String())
+}
+
+func (a *AlertManagerTime) UnmarshalJSON(jsonRepr []byte) error {
+	var serializedTime string
+	if err := json.Unmarshal(jsonRepr, &serializedTime); err != nil {
+		return err
+	}
+
+	t, err := time.Parse(time.RFC3339, serializedTime)
+	if err != nil {
+		return err
+	}
+
+	*a = AlertManagerTime(t)
+	return nil
 }
 
 func NewAlertmanager(hookURL string, proxyURL string, certPool *x509.CertPool) (*Alertmanager, error) {
@@ -75,18 +107,30 @@ func (s *Alertmanager) Post(ctx context.Context, event eventv1.Event) error {
 	labels["alertname"] = "Flux" + event.InvolvedObject.Kind + cases.Title(language.Und).String(event.Reason)
 	labels["severity"] = event.Severity
 	labels["reason"] = event.Reason
-	labels["timestamp"] = event.Timestamp.String()
 
 	labels["kind"] = event.InvolvedObject.Kind
 	labels["name"] = event.InvolvedObject.Name
 	labels["namespace"] = event.InvolvedObject.Namespace
 	labels["reportingcontroller"] = event.ReportingController
 
+	// The best reasonable `endsAt` value would be multiplying
+	// InvolvedObject's reconciliation interval by 2 then adding that to
+	// `startsAt` (the next successful reconciliation would make sure
+	// the alert is cleared after the timeout). Due to
+	// event.InvolvedObject only containing the object reference (namely
+	// the GVKNN) best we can do is leave it unset up to Alertmanager's
+	// default `resolve_timeout`.
+	//
+	// https://prometheus.io/docs/alerting/0.27/configuration/#file-layout-and-global-settings
+	startsAt := AlertManagerTime(event.Timestamp.Time)
+
 	payload := []AlertManagerAlert{
 		{
 			Labels:      labels,
 			Annotations: annotations,
 			Status:      "firing",
+
+			StartsAt: startsAt,
 		},
 	}