Attribute Customization - Calculated Value - Comment Timestamp

Hi

I need to have a custom attribute saving the timestamp when a comment is added.
I have a script that saves the timestamp, but it updates every time the Work Item is saved.

dojo.provide("com.saveCommentDate");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");

(function(){
    var WorkItemAttributes = com.ibm.team.workitem.api.common.WorkItemAttributes;
    var fromISOString = dojo.date.stamp.fromISOString;
    dojo.declare("com.saveCommentDate", null, {
        getValue: function(attribute, workItem, configuration) {
            var now = new Date();
            return dojo.date.stamp.toISOString(now, {milliseconds:true, zulu:true});
        }}
    );
})();

Anyone who knows how can I check if the update comes from a new comment?
Or, how to execute the script only on comment updates?
 Thanks!