Passing a Unix Timestamp to a form

Hi there. I'm trying to pass a UNIX timestamp to a form. From what I was reading, I can supposedly alter the value of a hidden field with the Gform field value $parameter name function. Here's what I wrote:

In my theme functions.php:
add_filter("gform_field_value_timestamp", "unixtime");
function unixtime($value){
return time();
}

In the form, the field named "timestamp" is hidden, the default value is blank, allow field to be populated dynamically is checked, and the value of the dynamic population is gform_field_value_timestamp.

Problem is, it's not populating at all.

What am I doing wrong?

Working!