Running a function every second
Timers are pretty simple to use.  There are only a few things to worry about and they are called out in the documentation.  From that page:


  "As long as you are using a Timer, you must keep a reference to it. As
  with any managed object, a Timer is subject to garbage collection when
  there are no references to it. The fact that a Timer is still active
  does not prevent it from being collected."


In your code, the reference to the Timer isn't stored anywhere.  This means it's eligible for garbage collection, and when that happens it will stop firing events.  So you need to store a reference somewhere, in your code it looks like a static field on the UserTimer class would make sense.

The other thing is if it's possible for the callback to run longer than the interval, you might want to detect this case and have the second callback not do any work.  If this is a possibility I'll have a line of code like:

if (!Monitor.TryEnter(timerLock))
    return;


Of course you need to release the monitor in a finally block in your callback.