The first bomb was dropped on the 6 August 1945 at 0815 when a single US B29 bomber nick-named the "Enola Gay" dropped an atomic bomb from an altitude of 9,000 metres on Hiroshima.

The second bomb was dropped on the 9 August 1945 at 1102 when another B29, this one known as "Bock's Car", released an atomic bomb over Nagasaki.

Source:
The Impact of the A-bomb, Hiroshima and Nagasaki, 1945-85 The Committee for the Compilation of Materials on Damage Caused by the Atomic Bombs in Hiroshima and Nagasaki. 1985.