Basically, yes such a timer can be created, but no you can't press a button to activate it. Whenever a timer has enough information, it's active. There's no way to enable or disable a timer then via the method Brady describes. Also, in order to edit a timer on the remote it must be located in the Macro side and can be the only function on that button (although the timer itself can point to a macro).

I can hear people thinking of that "is timergroup" setting in ProntoEdit. That's only for the creation of new buttons -- if a group is set to "timergroup", all blank buttons will default to a timer when edited. Otherwise they default to a macro.