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While the light from the cluster has taken about five billion years to reach us, the supernova itself exploded much earlier, nearly 10 billion years ago
http://www.sciencedaily.com/releases/2015/12/151216135824.htm
These multiple images of the galaxy presented a rare opportunity. As the matter in the cluster -- both dark and visible -- is distributed unevenly, the light creating each of these images takes a different path with a different length. Therefore the images of the host galaxy of the supernova are visible at different times.

Using other lensed galaxies within the cluster and combining them with the discovery of the Einstein Cross event in 2014, astronomers were able to make precise predictions for the reappearance of the supernova. Their calculations also indicated that the supernova appeared once before in a third image of the host galaxy in 1998 -- an event not observed by any telescope. To make these predictions they had to use some very sophisticated modelling techniques.

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