[MikeDelta]

There is only one event that happened, the question is how much time we will measure between that moment and the photons of the event reaching earth.

In the reference frame of earth it is 31 mio years. In frames that move faster and faster to the speed of light, relative to ours, this would take less and less time, an effect known as time dilation.

Obviously the article means that 3 hours after the first photons of this event reached earth (and we see it as happened), we managed to aim our sensors towards it and start measuring. It looks a bit clumsy indeed.

[sudhirj]

Another way to think about it would to change the distance and see if the sentence still makes sense. If the event was on the other side of the world, 100 light-milliseconds away, it wouldn't be an issue. If it happened on Mars, a light-minute or so away, also not an issue. If it happened on the Sun, 8 light-minutes away, this phrasing would also be fine, same for Jupiter and even Pluto. Not sure where the line is, but it's somewhere between 10 light-minutes and 31 million light-years, I guess :P