[Storebror]

The First World War: August the 4th 1914 (Well for Britain Any ways)

The first world war started with Austria-Hungary declaring war against Serbia at Juli 28th 1914.

The Second World War: September the 3rd 1939 (Well for Britain and France Any ways) - May 8th 1945 Europe

The raid against Poland started September 1st already.
WWII end date for europe depends on which time zone you're referring to. In the combat zone it was May 9th.

August the 15th 1945 (Or if you like September the 2nd 1945)

August 15th was a single-sided proclamation. This has been seen in europe before (where germany tried to surrender on the western front only) where it has been rejected. Hence September 2nd is the correct date.

Best regards - Mike