Possible to get Date value in form field?

I'm trying to submit a field in my form that includes today's date, but get errors (obviously) when trying to do something like this:

name: hidden_date
type: hidden
default: {{ now }}

What allows the "process" part of the form plugin to read values like from: "{{ form.value.email }}" but trying similar throws big no-no's? Thanks!

@thejame Have a look at Using Function Calls (data-*@)

Below is a working example.

I have done the following to get 'now' as default value in the Admin page of my plugin.

1. Create new plugin
   BASHCopy

   $ bin/plugin devtools new-plugin
   

2. In 'user/plugins/my-plugin/my-plugin.php' I added:
   TXTCopy

   public static function now()
   {
      return date('Y-m-d h:i:s', time());
   }
   

3. In '/user/plugin/my-plugin/blueprints.yaml' I defined the field as:
   YAMLCopy

   hidden_date:
    type: hidden
    data-default@: '\Grav\Plugin\MyPluginPlugin::now'
   

   In the admin page of the plugin, Grav generated the following hidden field.

   TXTCopy

   <input data-grav-field="hidden" data-grav-disabled="false" type="hidden" class="input" 
   name="data[hidden_date]" value="2018-07-21 08:36:37">