Modal Timer interval has no effect

I set up a modal timer, but the interval value doesn’t seem to change how often the timer fires.
Am I doing something wrong?

Here is the code:

class ModalTimerOperator5(bpy.types.Operator):
    """Operator which runs its self from a timer"""
    bl_idname = "wm.modal_update_panel"
    bl_label = "Timer-5"

    done_once = False
    _timer = None
    def modal(self, context, event):
        print ('timer')
        return {'PASS_THROUGH'}

    def execute(self, context):
        wm = context.window_manager
        self._timer = wm.event_timer_add(1.0, context.window)     # Interval not working.
        return {'RUNNING_MODAL'}

    def cancel(self, context):
        ModalTimerOperator5.done_once = False
        ModalTimerOperator5.timer_cycles = 0

        return {'CANCELLED'}