[slg]

A "day" has no meaning anywhere but on Earth. So if you are going to make the leap that the usage of primes is a sign of intelligence, you would also have to believe that this message was meant for us and only us. However the burst seems to originate from 3 billion light-years away so they would have had to start sending the message very shortly after life began on the Earth. Seems incredibly unlikely.

EDIT: Also an Earth day is not a constant value. It would have been much shorter 3 billion years ago and this cycle wouldn't be repeating in prime numbers. This theoretical alien intelligence would therefore need to not only be able to recognize life on Earth in the first few hundred million years it existed. It would also need to be able to accurately model the Earths orbit over the next 3 billion years in order to customize the message for when it would be received. All that work to send the message and they still needed to luck out that the 3 billion year journey happened to reach us at nearly the perfect time for our technology to advance enough so we could hear it.

[saas_sam]

It wouldn't have to do any of that if it was simply keeping an activity:non-activity ratio that resulted in 2 primes. I'm not sure to what degree of precision this flashing is happening but if it's very precise then there you go. 2 primes result no matter what your day/timescale/number system is.

[SketchySeaBeast]

But it's not, it's active for 90 days. You'd think they'd go for the trio.

[ncmncm]

Good one. It is rare that two primes can be added to get a third prime. And one of them is always 2. :-)

[SketchySeaBeast]

Admittedly, the options are more limited.

[ifdefdebug]

Also they would have had to infer our day cycle from observations originated 6 billion years ago, which would be strange because the age of the solar system is estimated to be 4.5b years.

[saalweachter]

Timed to match the slowing of the Earth's rotation, no less.