There is no safek about that. That depends on if you are European and call it Johrtzeit or American and call it Yartzeit. The safek is, if there will be any difference between Europeans and Americans at that time and how they get to E”Y.
What Yartzeit or Johrtzeit will be held after T’Chiyas Hamaisim? The of their own past death? The day of the Tchiya? Both?