gradle shadow plugin with build date in manifest

gistfile1.txt

buildscript {
        repositories {
                jcenter()
        }
        dependencies {
                classpath 'io.ratpack:ratpack-gradle:0.9.13'
                classpath 'com.github.jengelman.gradle.plugins:shadow:1.2.0'
        }
}

apply plugin: "io.ratpack.ratpack-java"
apply plugin: "com.github.johnrengelman.shadow"

version = '1.0'

mainClassName = 'com.acme.core.AppMain'

import java.text.SimpleDateFormat

shadowJar {
    mergeServiceFiles()
    manifest {
        attributes 'Built-Date': (new SimpleDateFormat("yyyy/MM/dd")).format(new Date()) //today's date
        attributes 'Built-By': System.getProperty('user.name')
        attributes 'Build-Jdk': System.getProperty('java.version')
        attributes 'Implementation-Title': project.name
        attributes 'Implementation-Version': project.version
        attributes 'Implementation-Vendor-Id': project.group
    }
    exclude 'server.log'
    exclude '*.log.gz'
}