For test/evaluation purposes only!
The official solution is to Activate the RDS/TS CAL License server and point the Server to License server with User/Device License.
But if you want to reset the timer and have another 120 days grace period, here is the solution:
Delete the REG_BINARY in
Leave the (Default) regkey
Note: you must take ownership and give admin users full control to be able to delete this key.
After a reboot the server should be working again for another 120 Days.