This script creates a shortcut (Alt+d) that adds a tag with the current date at the end of the card.

ginkgo_mousetrap.js

app.get('cards').models.map(function(c) {
    Mousetrap.bind('alt+d', function(e) {
        var today = new Date();
        var dd = today.getDate();
        var mm = today.getMonth()+1; //January is 0!
        var yyyy = today.getFullYear();

        if(dd<10) {
            dd='0'+dd
        } 

        if(mm<10) {
            mm='0'+mm
        } 

        today = '#'+yyyy+'-'+mm+'-'+dd;


        var id = app.get('cardId');
        var c = app.get('cards')._byId[id];
        c.saveContent(c.get('content') + '\n' + today);

        return false;
    });
});