This gist is for emberfire users. It's a Mixin that adds a property to DS.Model attributes that causes their value to be set to the special 'ServerValue.TIMESTAMP' on save.

server-timestamp-on-save.js

import Ember from 'ember';
import firebase from 'firebase';
const { Mixin, get, set } = Ember;

/**
 * A small Mixin for DS.Model for emberfire users that will set attributes that have
 * 'setToServerTimeStampOnSave' to the firebase 'ServerValue.TIMESTAMP' on save.
 * eg:
 *     clientUpdatedTime: attr('number', { setToServerTimestampOnSave: true }),
 */

export default Mixin.create({

    save() {
        get(this.store.modelFor(get(this, 'constructor.modelName')), 'attributes').forEach((value, key) => {
            if (value.options.setToServerTimestampOnSave && !get(this, 'isDeleted')) {
                set(this, key, firebase.database.ServerValue.TIMESTAMP);
            }
        });
        return this._super(...arguments);
    },

});