firestore-rest-server-timestamp-example.txt

POST https://firestore.googleapis.com/v1beta1/projects/firestore-fun/databases/(default)/documents:commit?key={YOUR_API_KEY}
 
{
 "writes": [
  {
   "update": {
    "name": "projects/firestore-fun/databases/(default)/documents/foo/bar",
    "fields": {
     "some-field": {
      "stringValue": "some-value"
     }
    }
   }
  },
  {
   "transform": {
    "document": "projects/firestore-fun/databases/(default)/documents/foo/bar",
    "fieldTransforms": [
     {
      "fieldPath": "servertime",
      "setToServerValue": "REQUEST_TIME"
     }
    ]
   }
  }
 ]
}