how append date build to versionNameSuffix on gradle

Also, do not forget to add import as Gradle first line:

import java.text.SimpleDateFormat;
...

beta {
    packageNameSuffix ".beta"
    versionNameSuffix "-beta" + "-build" + getDate()
    signingConfig signingConfigs.debug
}

def getDate() {
    def date = new Date()
    def formattedDate = date.format('yyyyMMddHHmmss')
    return formattedDate
}

Condensed:

def getDate() {
    return new Date().format('yyyyMMddHHmmss')
}

for simple one row solution define this property above android section 

final BUILD_DATE = new Date().format('yyyy_MM_dd_HHmm')

and then 

android {
    compileSdkVersion rootProject.ext.compileSdkVersion
    buildToolsVersion rootProject.ext.buildToolsVersion

    defaultConfig {
        applicationId APPLICATION_ID
        minSdkVersion rootProject.ext.minSdkVersion
        targetSdkVersion rootProject.ext.compileSdkVersion
        versionName GIT_TAG_NAME
        versionCode GIT_COMMIT_COUNT
        setProperty("archivesBaseName",`enter code here` "com-appname-$BUILD_DATE-$versionName")
    }
}

You can define in your build.gradle custom functions and variables.

def versionMajor = 3

def buildTime() {
    def df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm'Z'") // you can change it
    df.setTimeZone(TimeZone.getTimeZone("UTC"))
    return df.format(new Date())
}

Then you can use it:

android {
    defaultConfig {
       versionName "${versionMajor}-beta-build-${buildTime()}"
    }
}

or if you want to add it in you versionNameSuffix

beta {
    versionNameSuffix "-beta-build-${buildTime()}"      
}