Generate And Add An Unique Identifier To The Email

I am developing a html ordering system that does not use a database but sends the data via email. To differentiate between two orders I would like to generate and add an unique identifier to the email. One approach I have seen is using data object to generate an unique id:

function uniqid()
{
var newDate = new Date;
return newDate.getTime();
}

This is not guaranteed to be unique as two people can send their request at the exact same time. One question pops up, the data being generated, is this local or utc time? Also, the number being generated in this situation is quite big and not so easy to differentiate two orders for a human. Ideally I would like to generate a global sequence starting at 1 and incrementing by one for each user that clicks on the order button regardless of timing, timezone etc. Is this possible given the constraints given at the start of the post?