Quoting: Polson
In September 1939, Germany invaded Poland in cooperation with the Soviet Union, and war in Europe followed. The French and British did not declare war at first, hoping they could persuade Hitler through appeasement, but Hitler did not respond. The United Kingdom and France declared war.
September 1: German invasion of Poland. British give German Foreign Minister Joachim von Ribbentrop an ultimatum: all German forces out Poland or war will be declared.
September 3: 9am British ambassador to Berlin gives Nazi govt message "unless not later than 11 A.M., British Summer Time, to-day 3rd September, satisfactory assurances to the above effect have been given by the German Government and have reached His Majesty's Government in London, a state of war will exist between the two countries as from that hour."
September 3: 11am Britain declares war on Germany.
September 3: 12.03pm first British plane bound for German airspace airborne.
September 17: Soviet Union invades Poland.
Looks like the person writing that text for wikipedia knew **** all.