My expectation is that once started, if started again before it times out, a timer would restart from the default value, as per the description from the docs:
Starts or restarts a timer with the provided duration. If no duration is given, it will either restart with its initial value, or continue a paused timer with the remaining duration.
I set a test timer as below, and whilst a press of the ‘doorbell’ starts the timer, a second press does not restart it from it’s default value, and it times out.
When a timer is first started, the sensor shows as below:
However, if the timer is called again before timing out, the remaining time does not update to the preset value and once the first call is made and the preset time is reached the sensor shows as below and the time decrements, as if the timer has finished, which it has not:
It will stay this way until HA is restarted or automations are reloaded.
In all other respects, the timers work as described.