Yesterday, the best answer is: cos(x^2)^2.  (Contributed by dnialh. Good Job!)
dnialh solved the puzzle at 20:00:11.617; length: 4
asdf solved the puzzle at 20:00:34.434; length: 4
jabrams solved the puzzle at 20:03:30.403; length: 4
fire solved the puzzle at 20:23:24.582; length: 4
asdf solved the puzzle at 21:53:18.520; length: 29
asdf solved the puzzle at 21:54:00.764; length: 31
asdf solved the puzzle at 21:54:01.907; length: 31
asdf solved the puzzle at 21:54:08.255; length: 33
asdf solved the puzzle at 21:54:21.605; length: 34
asdf solved the puzzle at 21:54:24.935; length: 37
asdf solved the puzzle at 21:54:25.149; length: 37
asdf solved the puzzle at 21:54:28.141; length: 44
asdf solved the puzzle at 21:55:03.867; length: 26
asdf solved the puzzle at 21:55:11.801; length: 24
asdf solved the puzzle at 21:55:17.459; length: 23
asdf solved the puzzle at 21:55:55.769; length: 24
asdf solved the puzzle at 21:59:24.267; length: 24
asdf solved the puzzle at 22:00:56.971; length: 23
asdf solved the puzzle at 22:00:57.439; length: 23
asdf solved the puzzle at 22:00:57.650; length: 23
asdf solved the puzzle at 22:00:57.860; length: 23
asdf solved the puzzle at 22:00:58.064; length: 23
asdf solved the puzzle at 22:00:58.255; length: 23
asdf solved the puzzle at 22:00:58.448; length: 23
asdf solved the puzzle at 22:00:58.614; length: 23
asdf solved the puzzle at 22:00:58.826; length: 23
asdf solved the puzzle at 22:00:59.026; length: 23
asdf solved the puzzle at 22:00:59.231; length: 23
asdf solved the puzzle at 22:00:59.438; length: 23
asdf solved the puzzle at 22:00:59.645; length: 23
asdf solved the puzzle at 22:03:14.010; length: 24
m1o9n6s8t8e3r solved the puzzle at 08:48:29.645; length: 4