In Tenet the attack on the Opera happens the same day as the attack on the Stalsk-12 mine.
It seems to me there is no reason these two events should happen the same day, except that this simultaneity leads Sir Michael to link the Opera events with the Stalsk-12 mine and confirm Andrei's involvement. Michael said:
Two weeks ago, the same day as the Kiev opera siege, we spotted a detonation in northern Siberia just where Stalsk-12 was.
It looks like the simultaneity is only there to make the plot work. Is there another reason why the same date was chosen?