After the expiration of evaluation period of Window Server. you will face suddenly shutdown every one hour ! it’s annoying.therefore,in this case,you only have two options:

• Activate windows by purchasing a new Key.
• Extend evaluation period.(Simplest & Quickly method).

Therefore, to extend the evaluation period follow the bit mentioned steps below:

• Open Command prompt as Administrator

• Type the following command “slmgr.vbs /rearm” which reset the licensing status of machine and press enter

• Restart your server then you will found the evaluation period expired to 180 day

Thanks to MOHAMMED EL KASSAS

https://blog.citrix24.com/upgrade-windows-2012-r2-evaluation-full-version/

https://mohamedelkassas.wordpress.com/author/mohamedelkassas/

https://mohamedelkassas.wordpress.com/2015/09/04/evaluation-period-expired-for-windows-server-2012-r2-how-to-extend-it/