Form select option results in "Illegal Choice has been detected"
That's because the form gets generated both upon output and submission, and you're using form values that are time-dependent. When you first output the form it generates options like this:

1313615248
1314220048
1314824848
1315429648


And when a user submits the form, it will generate new values, with a few second difference:

1313615268
1314220068
1314824868
1315429668
(+20 second difference)


So none of the previous options are present here. Drupal's validator by default checks if the submitted option is among generated options.