Note that doing this would change the class semantics.
Timer "[...] represents an action that should be run only after a 
certain amount of time has passed — a timer." and the example clearly 
shows that the action is run *once*. 

Timer is basically an example of how to write custom Thread 
subclasses; if you want a "repetitive action", you may easily write a 
subclass based on the code you posted. Not every useful class must be 
in the standard library...