Windows Trial/Evaluation period expires after 180 days. After that Windows Trial/Evaluation period can be extended for next 180 days:
1. Login to your Windows RDP connection.
2. Run PowerShell app.
3. Run the following command in PowerShell command line:slmgr -rearm
4. Reboot server.
Please note, that if not fully activated, after Trial/Evaluation period expiration Windows starts to shutdown automatically once in a hour. It may be prevented by extending Trial/Evaluation period for next 180 days.