Triggers is just using a function editor with javascript code. So if you want to add a date, you should be able to create one with “new Date()” and use the javascript methods here if you want to store the date as a string: Date Methods Reference
You can also just store the field as a Date which is supported in MongoDB and is most often the better thing to do when storing a date in the database. Your client-side application can be in charge of how it wants to convert a date into a string