Bond prices when rates immediately fall/rise

I have seen various questions that state, ‘the rates immediately drop/rise after initial purchase.’ In one place, the bond was revalued using the new rates at (t+1), t being date of initial purchase. Elsewhere, I see that the bond price is recalculated at time t. Which one is more preferred?

1 Day is not going to make a difference, so just assume t. that’s my take.