From the first episode of Death Note, the protagonist reads the rules inside the Death Notebook:
“40秒以内に死因を書くとそのとおりになる”
Later, immediately after writing someone's name in the book, he remarks:
40秒で心臓麻痺だったな…
Question: Why is the past tense of the copula (だった) used here, instead of the present tense (だ)? (Given that the person hasn't yet died, and he's wondering what's going to happen)
