Limits of evaluation versions Windows Server

All evaluation versions are fully functional during the evaluation period, although booting to Safe mode is not available. The Windows Server Standard and Windows Server Datacenter editions come with the activation key pre-installed. After the 180-day evaluation period elapses, the server warns you in various ways depending on the edition:
Windows Server Standard; Windows Server Datacenter:

• The following warning appears on the Desktop: Windows License is expired
• When you log on to Windows, you are prompted with the following options:
  - Activate now
  - Ask me later
• The system shuts down every hour.
• The only updates that can be installed are security updates.
• Event ID 100 from source WLMS “The license period for this installation of Windows has expired. The operating system will shut down every hour.” appears in the Application log.

Windows Server Essentials: you receive warnings on the Desktop and on the dashboard, but the server does not shut down.
Information from microsoft.com