When a timer is started and expires, the timerHandle is left in place. It can be cleared by the Reset action.
What this means to a novice user of the gateway / Timer is that once a timer is used in a rule, it can’t be reused until the Add-ons are reloaded or the gateway is restarted.
I’m not sure whether it is better to change the behaviour to clear the timerHandle or to change the documentation to make it more obvious to a noob like me.