What did you do?
I have created alerts in alertmanager via POST /api/v2/alerts, explicitly setting the startsAt & endsAt values. When the alerts are requested at GET /api/v2/alerts, both are shown correctly. When alertmanager reports those alerts to a custom webhook, the received endsAt is:

• If the alert is not resolved: 0001-01-01T00:00:00Z.
• If the alert is resolved: The resolved date.

What did you expect to see?
I expect to receive the same values in the webhook as received when requesting the alerts manually to GET /api/v2/alerts, regardless of the status of the alert.

What did you see instead? Under which circumstances?
I'm receiving different values depending if the alerts are resolved or not, while when requesting the alerts via GET /api/v2/alerts, the endsAt date is always present with a "non-empty" value.
This makes impossible to retrieve a informative endsAt while using send_resolved: false.

Environment

• System information:

  Linux 5.15.0-69-generic x86_64
  

• Alertmanager version:

  alertmanager, version 0.25.0 (branch: HEAD, revision: 258fab7cdd551f2cf251ed0348f0ad7289aee789)
    build user:       root@abe866dd5717
    build date:       20221222-14:51:36
    go version:       go1.19.4
    platform:         linux/amd64
  

  Also reproduced using this docker image

• Prometheus version:

  Not using prometheus.

• Alertmanager configuration file:

  global:
      resolve_timeout: 10m
  
  route:
      group_by: ['...']
      group_wait: 1s
      group_interval: 1s
      repeat_interval: 1s
      receiver: "log"
  
      routes: []
  
  receivers:
  - name: "log"
    webhook_configs:
    - url: http://localhost:10080/notify
      send_resolved: false

Steps to reproduce

Alertmanager setup:

• Copy the configuration file to your current directory.
• Run alertmanager in the container:

  docker run -d --net=host --name prometheus-alertmanager-container -e TZ=UTC -v $(pwd)/alertmanager.yml:/etc/alertmanager/alertmanager.yml ubuntu/prometheus-alertmanager:0.23-22.04_beta
  

• Verify the configuration file is correcly loaded: curl localhost:9093/api/v2/receivers
  The output should be: [{"name":"log"}]

Webhook setup:

• Copy the following code to a file in the current directory named notification-logger.py.

  from flask import Flask, request, Response
  import json
  
  app = Flask(__name__)
  @app.route('/notify', methods=['POST'])
  def log_notification():
          notification = request.json
          print(json.dumps(notification, indent=2))
          return Response('OK', status=200)
  
  
  app.run(host='localhost', port=10080)

• Run the webhook server: python3 notification-logger.py.

Create & check alerts:

• Create an alert (you may need to modify the value in endsAt to provide a future date):

  curl -XPOST localhost:9093/api/v2/alerts -H 'Content-type: application/json' -d '[{"startsAt": "2023-04-27T12:23:33.414Z", "endsAt": "2023-04-28T12:23:32.414Z", "generatorURL": "http://some.domain.com/some/path", "annotations": {}, "labels": {"alertname": "test-alert", "severity": "warning"}}]'
  

• Retrieve the alert via GET /api/v2/alerts:

  curl -XGET localhost:9093/api/v2/alerts 2>/dev/null | python3 -m json.tool
  

  The output should be:

  [
      {
          "annotations": {},
          "endsAt": "2023-04-28T12:23:32.414Z",
          "fingerprint": "41f99308f4b244ea",
          "receivers": [
              {
                  "name": "log"
              }
          ],
          "startsAt": "2023-04-27T12:23:33.414Z",
          "status": {
              "inhibitedBy": [],
              "silencedBy": [],
              "state": "active"
          },
          "updatedAt": "2023-04-27T10:47:35.242Z",
          "generatorURL": "http://some.domain.com/some/path",
          "labels": {
              "alertname": "test-alert",
              "severity": "warning"
          }
      }
  ]

  Notice how the value in endsAt is the same as provided in the POST request.
• Check out the log in the webhook server. It should be the following:

  {                                                                                                                                                                                                                                    
    "receiver": "log",                                                                                                                                                                                                                 
    "status": "firing",                                                                                                                                                                                                                
    "alerts": [                                                                                                                                                                                                                        
      {                                                                                                                                                                                                                                
        "status": "firing",                                                                                                                                                                                                            
        "labels": {                                                                                                                                                                                                                    
          "alertname": "test-alert",                                                                                                                                                                                                   
          "severity": "warning"                                                                                                                                                                                                        
        },                                                                                                                                                                                                                             
        "annotations": {},                                                                                                                                                                                                             
        "startsAt": "2023-04-27T12:23:33.414Z",                                                                                                                                                                                        
        "endsAt": "0001-01-01T00:00:00Z",                                                                                                                                                                                              
        "generatorURL": "http://some.domain.com/some/path",                                                                                                                                                                            
        "fingerprint": "41f99308f4b244ea"                                                                                                                                                                                              
      }                                                                                                                                                                                                                                
    ],                                                                                                                                                                                                                                 
    "groupLabels": {                                                                                                                                                                                                                   
      "alertname": "test-alert",                                                                                                                                                                                                       
      "severity": "warning"                                                                                                                                                                                                            
    },                                                                                                                                                                                                                                 
    "commonLabels": {                                                                                                                                                                                                                  
      "alertname": "test-alert",                                                                                                                                                                                                       
      "severity": "warning"                                                                                                                                                                                                            
    },                                                                                                                                                                                                                                 
    "commonAnnotations": {},                                                                                                                                                                                                           
    "externalURL": "http://ubnt:9093",                                                                                                                                                                                                 
    "version": "4",                                                                                                                                                                                                                    
    "groupKey": "{}:{alertname=\"test-alert\", severity=\"warning\"}",                                                                                                                                                                 
    "truncatedAlerts": 0                                                                                                                                                                                                               
  }  

  Notice how the endsAt value in the alert is 0001-01-01T00:00:00Z instead of the provided value.