We've had the ability to block the sun for quite some time. It wouldn't be surprising to find out there are massive rolls of metal foil already in position at the L1 point just waiting for the signal to unroll. Think of all those "secret military" launches we've conducted over the last 70 years. Lagrange points are areas of gravitational equilibrium between planets/moons/the sun. L1 is between earth and the sun. If "they" decided to shut off the sun, "they" could do it at any time. How long would it take to freeze the earth and kill 8 billion useless eaters? I would guess a few weeks, but I posed this question a year or two ago and the consensus was that it would take a lot longer due to the oceans retaining their heat and the molten interior. I still think "they" can do it anytime "they" want.