Update Build - Assemble date with workflow

I'm trying to set the build number assemble date to current date when a release issue becomes Fixed.

The value assignment doesnt seem to be working

exports.rule = entities.Issue.onChange({

// TODO: give the rule a human-readable title
title: 'Set release build date',
guard: function(ctx) {
   return ctx.issue.fields.becomes(ctx.Resolution, ctx.Resolution.Fixed);
},
action: function(ctx) {
   if (ctx.issue.fields.type.name === 'Release') {
       var tempbuild = ctx.fixedinbuild.values.find(function(fixedinbuild) {
           return fixedinbuild.name === ctx.issue.summary;
       });
       tempbuild.assembleDate = Date.now();
}