The JN-25 Naval code was not broken until January 1942. Prior to December 7th,1941 most US codebreakers were concentrating on the “Purple” diplomatic code. The naval code was a lower priority. Also, The Imperial Japanese Navy had changed the code in the fall of 1941. So JN-25 was only about 3% broken in early December 1941. After the attacks on Hawaii, the Phillippines and Malaya, priorities shifted and the code was mostly broken by January.
Besides, the attack plans were kept under the strictest security and Kido Butai was under radio silence until the attack was launched.
To Greenlantern, detailed information about the time and date of the attack might very well have been intercepted thru non-navel codes. The radio silence only applied to the fleet after it sailed to attack Pearl. Japanese diplomats and military leaders were aware in advance of the attack and war declaration and told to prepare for a war footing.
Plenty of time and sources for FDR to scheme our entry into the war.