app.js

import DateHelpers from './helpers/DateHelpers'
...
app.use(DateHelpers);

inside DateHelpers.js

import moment from 'moment';
import 'moment/min/locales';

export default {
    install: (app, options) => {
        app.mixin({
            methods: {
                ago: function (date) {
                    return moment(date).fromNow();
                },
            },
        })
    },
};

now we can use ago() function inside our components