This is according to then-Chairman of the Joint Chiefs of Staff, General Hugh Shelton.
"the codes were actually missing for months. [...] That's a big deal -- a gargantuan deal."
Shelton says that this occurred in 2000. However, Ret. Air Force Lt. Col Robert Patterson has a different story. According to him, the president misplaced the codes in the White House the day after the Monica Lewinsky scandal broke in 1998.