Add Execution Date

I need to create a new environment variable to add it to jenkins and this allows me to use it within any script or HTML content to update the start date of a job compilation.

I am defining in the global configuration of Jenkins creating new variable, within Environment variables I add the name and value of the variable to create

Also (through plugin) I define an Inject environment variables to the build process, this I do within the configuration of a job.

But when I put the value of the created variable, this does not bring me the result of the value of the variable, it only brings me the same Code without changes.
Texto en cursiva

Finally the content that I add comes to me in this way, it does not throw values such as the date of execution of compilation. Am I doing something wrong?
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TNKS

There is no support for defining environment variables that way as far as I know.

I understand, but there is no way I can configure for the content of the email notification to inform me of the start and end time of a build. TNKS