I do not quite understand the question description
Any time, you will only know for certain at the 15 minute mark whether there was poison.
Therefore you can't feed a pig at 00:01, again at 00:02 and determine the deadly bucket by the second of th pig's death.
@czhangaegean It is the latter case since the problem states that the pig dies within 15 minutes.
And this is actually a very critical condition to impact how you can design the feeding solution. With only the time window, it means you don't want to feed a pig multiple times within 15 minutes, because the worst scenario is that the pig dies and no way to tell which bucket causes the death.