It Only SEEMED The World Was Ending (-SPOILER-)
SPOILER...
The world was not ending...it only seemed that way to Patrick after his girlfriend died. He said to Sandra, When she died, "they said the world would end." His world had ended, or he was considering ending it. He was a closed, insular person again, until he met Sandra. His world began anew when he met Sandra. For the first time since his girlfriend's death, he felt life. When Patrick and Sandra kissed, the blinding light was the end of his Doomsday world...his new life was beginning with a new love.
This is not a highly factual analysis (if you want that, I cite to Mystery Science Theater 3000, "you should really just relax.") This is an allegorical interpretation; ONE level where it worked for me.
One edit: I just re-saw the film. There is a countdown at the end. But when the countdown reaches "0", the world does not end. The world continues for at least another 15 seconds while Patrick and Sandra give up their mutual suicide pact and instead have a lingering kiss. To me this is a clue that the world did not end. It seemed like it was ending to Patrick. But when he found love again in his heart, the world did not end after all. It's just a sign - to me - and I realize that a lot of people will not share this interpretation.