With the -poll flag the time-server will periodically poll the time and return the polled value in queries. Otherwise, the time server will always return the current time in queries. Polled mode offers better performance when the time server is queried at a high frequency, but does not support events.
Connected clients can query the virtual time, install alarm events that are triggered after a certain amount of virtual time, or install keepalive events that trigger at a periodic rate (real time).