I've been trying to get my head around this over the last few days. Still not sure I'm there yet.
It seems to me that the effect of fiscal policy at the Zero Lower Bound in New Keynesian models is extremely sensitive to timing the exit. This isn't about finding "shovel ready" projects that can be started relatively quickly; this is about finding projects that can be stopped instantly.
"Better late than never" is OK for starting fiscal policy; it is not OK for stopping fiscal policy. If we believe simple NK models.