How to get build time stamp from Jenkins build variables?

Build Timestamp Plugin will be the Best Answer to get the TIMESTAMPS in the Build process.

Follow the below Simple steps to get the "BUILD_TIMESTAMP" variable enabled.

STEP 1:

Manage Jenkins -> Plugin Manager -> Installed...
Search for "Build Timestamp Plugin".
Install with or without Restart.

STEP 2:

Manage Jenkins -> Configure System.
Search for 'Build Timestamp' section, then Enable the CHECKBOX.
Select the TIMEZONE, TIME format you want to setup with..Save the Page.

USAGE:

When Configuring the Build with ANT or MAVEN, 
Please declare a Global variable as, 
E.G.  btime=${BUILD_TIMESTAMP}
(use this in your Properties box in ANT or MAVEN Build Section)

use 'btime' in your Code to any String Variables etc..

NOTE: This changed in Jenkins 1.597, Please see here for more info regarding the migration

You should be able to view all the global environment variables that are available during the build by navigating to https://<your-jenkins>/env-vars.html.

Replace https://<your-jenkins>/ with the URL you use to get to Jenkins webpage (for example, it could be http://localhost:8080/env-vars.html).

One of the environment variables is :

BUILD_ID
    The current build id, such as "2005-08-22_23-59-59" (YYYY-MM-DD_hh-mm-ss)

If you use jenkins editable email notification, you should be able to use ${ENV, var="BUILD_ID"} in the subject line of your email.