index.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.insertServerTime = functions.database.ref('/users/{deviceId}/tData/{key}').onWrite(event => {
    const originalData = event.data
    const serverTime = {time: event.timeStamp}
    console.log('update key ->' + event.params.key)

    return admin.database().ref('/users/'+event.params.deviceId+'/tData/' + event.params.key).push(serverTime)
    //return event.data.ref.parent.push().set(serverTime)
})