Add timestamp in Firestore documents

REALTIME SERVER TIMESTAMP USING FIRESTORE

import firebase from "firebase/app";

const someFunctionToUploadProduct = () => {

       firebase.firestore().collection("products").add({
                name: name,
                price : price,
                color : color,
                weight :weight,
                size : size,
                createdAt : firebase.firestore.FieldValue.serverTimestamp()
            })
            .then(function(docRef) {
                console.log("Document written with ID: ", docRef.id);
            })
            .catch(function(error) {
                console.error("Error adding document: ", error);
            });

}

All you need is to import 'firebase' and then call firebase.firestore.FieldValue.serverTimestamp() wherever you need it. Be careful with the spelling though, its "serverTimestamp()". In this example it provides the timestamp value to 'createdAt' when uploading to the firestore's product's collection.

For Firestore

ref.doc(key).set({
  created: firebase.firestore.FieldValue.serverTimestamp()
})

Use firestore Timestamp class, firebase.firestore.Timestamp.now().

Since firebase.firestore.FieldValue.serverTimestamp() does not work with add method from firestore. Reference

firebase.firestore.FieldValue.serverTimestamp()

Whatever you want to call it is fine afaik. Then you can use orderByChild('created').

I also mostly use firebase.database.ServerValue.TIMESTAMP when setting time

ref.child(key).set({
  id: itemId,
  content: itemContent,
  user: uid,
  created: firebase.database.ServerValue.TIMESTAMP 
})