As in a one-off lose of 18 minutes or a one-off 18 minute gain, or extending/shortening the day/week/year by 18 minutes permanently? What's the final effect you're going for here?

@Ash Good catch! I didn't mean for that to be confusing, I just meant a one-off. Not permanent. I'll update.

Also what does "slowed down" mean? If time goes 50% slower for 18 minutes (of "real" time), it would have no effect. Should the perception of time go slower instead maybe? Or should only humans or non-living organisms be affected by it?

@Battle Ideally both. The day the aliens choose must slow down by 18 minutes of "real time." That's the requirement. The quality metric would favor an approach that is also perceptible. If not directly perceptible some clues that they were pranked would play into the quality metric too.

Something something Alcubierre something general relativity mumble redshift...

Possible duplicate of Plausible way to make a planet spin faster

Known physics and aliens playing around in our system sort of contradict each other. Are we to assume they travelled here by slow boat for thousands of years just to play a prank on us? And if they have FTL, time manipulation is basically a solved problem.

For the record, the amount of energy involved is so large that even if you decided to take one year to play the prank the crust of the Earth would melt. Multiple times. Relevant XKCD What If.

Alien practical jokes against Earth.... Sounds like the Great Attractor is at it again. I could almost be convinced to give this its own tag (alongside santa-claus).

@ArashHowaida You're saying time would slow by 18 minutes, but you haven't said "compared to what" or how people on Earth would even know it.

That's simple. Planet wide 18-minute Pan-Galatic Gargle Blaster.

You're leading up to the gap in the Nixon tapes, right?

The problem with altering the conditions of something that you live not "in" but "within" is that you just can't notice any change. You need an external observer to be able to notice the change. Just like you don't notice the Earth spinning, but you need an external reference (other planets) to notice it.

Are you actually looking for a gradual process? It seems to me that hacking into the atomic clocks and generating an simultaneous 18-minute jump in all of them would be sufficiently noticeable. (Or perhaps only most of them, for added confusion.)

@MontyHarder - "you haven't said "compared to what"" - Itself surely? I see it as a car travelling for 2 hours. First hour it travels at 60 miles per hour and second hour OP wants to slow it down by 18 mph so we make it travel at 42 mph instead. Except here it is time itself, so instead of time travelling at 60 minutes per hour he wants it slowed down by 18 minutes, so the second hour would be (rather unintuitively!) 78 minutes per hour. Expand this out to the other criteria (over a 24hr period) and you're set. Or have I missed something?

@RyanfaeScotland but if I'm inside that hour, the minute hand on my watch still goes round exactly once - so in what sense did it "really" take 78 minutes? Compared to a clock on the moon, or orbiting Betelgeuse, or what? At home on Earth I'm experiencing time at the rate it elapses, so what specifically would look different?

@Useless - If you look at Monty's comment again you'll see it has 2 questions and I feel like your comment is more related to his second question where as my comment was related just to the first (which I reckoned was simpler). I don't have time to address the second at the moment and also I feel like an answer to that would actually broach more on the side of an Answer to OPs question than a Comment and doesn't belong here. However, I will say that in this case I'd believe a Universal Truth, if we say 78 minutes have elapsed, 78 minutes have elapsed. How we prove it is another question!