diff --git a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesCreateEventHub.json b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesCreateEventHub.json
index 70ada05d2e07..7e26c29ba56a 100644
--- a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesCreateEventHub.json
+++ b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesCreateEventHub.json
@@ -14,7 +14,10 @@
         "eventHubName": "ehn",
         "keyName": "managementKey",
         "sharedAccessKey": "someSecretvalue",
-        "timestampPropertyName": "someTimestampProperty"
+        "timestampPropertyName": "someTimestampProperty",
+        "ingressStartAt": {
+          "type": "EarliestAvailable"
+        }
       }
     },
     "api-version": "2020-05-15"
@@ -35,7 +38,10 @@
           "eventHubName": "ehn",
           "keyName": "managementKey",
           "provisioningState": "Succeeded",
-          "creationTime": "2017-04-18T19:20:33.2288820Z"
+          "creationTime": "2017-04-18T19:20:33.2288820Z",
+          "ingressStartAt": {
+            "type": "EarliestAvailable"
+          }
         }
       }
     },
@@ -54,7 +60,10 @@
           "eventHubName": "ehn",
           "keyName": "managementKey",
           "provisioningState": "Succeeded",
-          "creationTime": "2017-04-18T19:20:33.2288820Z"
+          "creationTime": "2017-04-18T19:20:33.2288820Z",
+          "ingressStartAt": {
+            "type": "EarliestAvailable"
+          }
         }
       }
     }
diff --git a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesCreateEventHubWithCustomEnquedTime.json b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesCreateEventHubWithCustomEnquedTime.json
new file mode 100644
index 000000000000..67397fde8632
--- /dev/null
+++ b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesCreateEventHubWithCustomEnquedTime.json
@@ -0,0 +1,74 @@
+{
+  "parameters": {
+    "subscriptionId": "subid",
+    "resourceGroupName": "rg1",
+    "environmentName": "env1",
+    "eventSourceName": "es1",
+    "parameters": {
+      "location": "West US",
+      "kind": "Microsoft.EventHub",
+      "properties": {
+        "eventSourceResourceId": "somePathInArm",
+        "serviceBusNamespace": "sbn",
+        "consumerGroupName": "cgn",
+        "eventHubName": "ehn",
+        "keyName": "managementKey",
+        "sharedAccessKey": "someSecretvalue",
+        "timestampPropertyName": "someTimestampProperty",
+        "ingressStartAt": {
+          "type": "CustomEnqueuedTime",
+          "time": "2017-04-01T19:20:33.2288820Z"
+        }
+      }
+    },
+    "api-version": "2020-05-15"
+  },
+  "responses": {
+    "200": {
+      "body": {
+        "id": "/subscriptions/subid/resourceGroups/rg1/providers/Microsoft.TimeSeriesInsights/Environments/env1/eventSources/es1",
+        "location": "West US",
+        "name": "es1",
+        "type": "Microsoft.TimeSeriesInsights/Environments/EventSources",
+        "tags": {},
+        "kind": "Microsoft.EventHub",
+        "properties": {
+          "eventSourceResourceId": "somePathInArm",
+          "serviceBusNamespace": "sbn",
+          "consumerGroupName": "cgn",
+          "eventHubName": "ehn",
+          "keyName": "managementKey",
+          "provisioningState": "Succeeded",
+          "creationTime": "2017-04-18T19:20:33.2288820Z",
+          "ingressStartAt": {
+            "type": "CustomEnqueuedTime",
+            "time": "2017-04-01T19:20:33.2288820Z"
+          }
+        }
+      }
+    },
+    "201": {
+      "body": {
+        "id": "/subscriptions/subid/resourceGroups/rg1/providers/Microsoft.TimeSeriesInsights/Environments/env1/eventSources/es1",
+        "location": "West US",
+        "name": "es1",
+        "type": "Microsoft.TimeSeriesInsights/Environments/EventSources",
+        "tags": {},
+        "kind": "Microsoft.EventHub",
+        "properties": {
+          "eventSourceResourceId": "somePathInArm",
+          "serviceBusNamespace": "sbn",
+          "consumerGroupName": "cgn",
+          "eventHubName": "ehn",
+          "keyName": "managementKey",
+          "provisioningState": "Succeeded",
+          "creationTime": "2017-04-18T19:20:33.2288820Z",
+          "ingressStartAt": {
+            "type": "CustomEnqueuedTime",
+            "time": "2017-04-01T19:20:33.2288820Z"
+          }
+        }
+      }
+    }
+  }
+}
diff --git a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesGetEventHub.json b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesGetEventHub.json
index 610270bd8d01..8f36cdfadf48 100644
--- a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesGetEventHub.json
+++ b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesGetEventHub.json
@@ -22,7 +22,10 @@
           "eventHubName": "ehn",
           "keyName": "managementKey",
           "provisioningState": "Succeeded",
-          "creationTime": "2017-04-18T19:20:33.2288820Z"
+          "creationTime": "2017-04-18T19:20:33.2288820Z",
+          "ingressStartAt": {
+            "type": "EarliestAvailable"
+          }
         }
       }
     }
diff --git a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesListByEnvironment.json b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesListByEnvironment.json
index 09aae4112733..5201d9baafcf 100644
--- a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesListByEnvironment.json
+++ b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesListByEnvironment.json
@@ -23,7 +23,10 @@
               "eventHubName": "ehn",
               "keyName": "managementKey",
               "provisioningState": "Succeeded",
-              "creationTime": "2017-04-18T19:20:33.2288820Z"
+              "creationTime": "2017-04-18T19:20:33.2288820Z",
+              "ingressStartAt": {
+                "type": "EarliestAvailable"
+              }
             }
           }
         ]
diff --git a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesPatchTags.json b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesPatchTags.json
index 0473904851c2..fc2c6b53920c 100644
--- a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesPatchTags.json
+++ b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/examples/EventSourcesPatchTags.json
@@ -30,7 +30,10 @@
           "keyName": "managementKey",
           "provisioningState": "Succeeded",
           "creationTime": "2017-04-18T19:20:33.2288820Z",
-          "timestampPropertyName": "someOtherTimestampProperty"
+          "timestampPropertyName": "someOtherTimestampProperty",
+          "ingressStartAt": {
+            "type": "EarliestAvailable"
+          }
         }
       }
     }
diff --git a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/timeseriesinsights.json b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/timeseriesinsights.json
index 4bd8ce2a695c..f41cffee5c2c 100644
--- a/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/timeseriesinsights.json
+++ b/specification/timeseriesinsights/resource-manager/Microsoft.TimeSeriesInsights/stable/2020-05-15/timeseriesinsights.json
@@ -355,6 +355,9 @@
         "x-ms-examples": {
           "CreateEventHubEventSource": {
             "$ref": "./examples/EventSourcesCreateEventHub.json"
+          },
+          "EventSourcesCreateEventHubWithCustomEnquedTime": {
+            "$ref": "./examples/EventSourcesCreateEventHubWithCustomEnquedTime.json"
           }
         },
         "description": "Create or update an event source under the specified environment.",
@@ -1410,7 +1413,6 @@
       "properties": {
         "kind": {
           "type": "string",
-          "readOnly": true,
           "description": "The kind of the environment.",
           "enum": [
             "Gen1",
@@ -1429,6 +1431,9 @@
           "description": "Key-value pairs of additional properties for the environment."
         }
       },
+      "required": [
+        "kind"
+      ],
       "description": "Parameters supplied to the Update Environment operation."
     },
     "Gen1EnvironmentUpdateParameters": {
@@ -1843,6 +1848,29 @@
         }
       }
     },
+    "IngressStartAtProperties": {
+      "type": "object",
+      "description": "An object that contains the details about the starting point in time to ingest events.",
+      "properties": {
+        "type": {
+          "type": "string",
+          "description": "The type of the ingressStartAt, It can be \"EarliestAvailable\", \"EventSourceCreationTime\", \"CustomEnqueuedTime\".",
+          "enum": [
+            "EarliestAvailable",
+            "EventSourceCreationTime",
+            "CustomEnqueuedTime"
+          ],
+          "x-ms-enum": {
+            "name": "IngressStartAtType",
+            "modelAsString": true
+          }
+        },
+        "time": {
+          "type": "string",
+          "description": "ISO8601 UTC datetime with seconds precision (milliseconds are optional), specifying the date and time that will be the starting point for Events to be consumed."
+        }
+      }
+    },
     "EventSourceCreateOrUpdateParameters": {
       "discriminator": "kind",
       "properties": {
@@ -1915,7 +1943,6 @@
       "properties": {
         "kind": {
           "type": "string",
-          "readOnly": true,
           "description": "The kind of the event source.",
           "enum": [
             "Microsoft.EventHub",
@@ -1934,6 +1961,9 @@
           "description": "Key-value pairs of additional properties for the event source."
         }
       },
+      "required": [
+        "kind"
+      ],
       "description": "Parameters supplied to the Update Event Source operation."
     },
     "EventHubEventSourceUpdateParameters": {
@@ -2046,6 +2076,11 @@
         "timestampPropertyName": {
           "type": "string",
           "description": "The event property that will be used as the event source's timestamp. If a value isn't specified for timestampPropertyName, or if null or empty-string is specified, the event creation time will be used."
+        },
+        "ingressStartAt": {
+          "x-ms-client-flatten": true,
+          "$ref": "#/definitions/IngressStartAtProperties",
+          "description": "An object that contains the details about the starting point in time to ingest events."
         }
       },
       "allOf": [