On 11 December 1941, four days after the Japanese attack on Pearl Harbor and the United States declaration of war against the Japanese Empire, Nazi Germany declared war against the United States, in response to what was claimed to be a series of provocations by the United States government when the US was still officially neutral during World War II. The decision to declare war was made by Adolf Hitler, apparently offhand, almost without consultation. Later that day, the United States declared war on Germany.

Based on this passage, when did the united states declare war on germany? Let's give stream of consciousness first:
To answer the question, consider the following: Later that day, the United States declared war on Germany.
So, the final answer is 11 December 1941.