What time did the United States of America enter World War II?

December 7th 1941 war was declared against Japan

December 11th 1941 war was declared against Germany & Italy

1941, when the US entered the war because Pearl harbour was attacked December 7, 1941.

In terms of a Declaration of War, that was on December 8, 1941(the day after the Japanese attack on Pearl Harbor). The U.S. Congress declared war on Japan. The U.S. had been sending aid and supplies to Britain for some time before that and also through the Lend Lease Act naval ships were sent. Also the U.S. Navy had been providing escort to convoys across the Atlantic as protection against Nazi U-boats(submarines). There were also attacks against U.S. ships in the Atlantic with at least one Naval ship(maybe more) being sunk by U-boats. This was before Pearl Harbor, so it was a steadily escalating situation with regard to America becoming fully involved.

The U.S. entered World War II in 1941, the day after the Japanese surprise attack at the naval base at Pearl Harbor, Hawaii.

But be aware that World War II began two years earlier, in Europe in 1939.
The United States entered WW II in 1941 we only were in battle for 4 years