http://www.users.bigpond.com/pacificwar/AtomBomb_Japan.html
On 4 August 1945, American aircraft dropped leaflets on Hiroshima warning
the citizens to expect terrible destruction to be visited upon their city because
Japan had refused to surrender. Although many civilians had already been
evacuated to the country, this warning was largely ignored. On August 6, the
first atomic bomb was dropped on this city. At Hiroshima, 60,000 Japanese
died and a similar number were injured.

アメリカ時間の8月4日(日本時間8月3日）に投下予告したという記述が
多いようだな。