Gist for medium article https://medium.com/evenbit/firebase-cloud-functions-on-the-clock-312d7c14327

index.ts

import * as functions from "firebase-functions";
import * as admin from "firebase-admin";

admin.initializeApp(functions.config().firebase);

export const onCreated = functions.database.ref("timings/{id}/created").onWrite(event => {
    return event.data.ref.parent.child("handled").set(admin.database.ServerValue.TIMESTAMP);
});

export const onHandled = functions.database.ref("timings/{id}").onWrite(async event => {
    const entry = event.data.val();
    if (!entry.handled) {
        return;
    }
    return admin.database().ref("durations").child(event.params.id).set(entry.handled - entry.created);
});