Dates are not stored in mysql as 2001-01-12 like you have. They instead use the basic unix timestamp and base caluclations off of that.
So what to do:
use PHP's mktime() function
Usage: mktime(hour, minute, second, month, day, year)
So in your case:
$datevalue = mktime(0, 0, 0, 1, 12, 2001);
and then modify your SQL to read:
values ('$datevalue')
That's if you want to set your timestamp to some specified, fixed date.
If, however, you just want to automatically include a timestamp for the current date and time, use this method instead:
$datevalue = date("Y-m-d");
and that will set your datevalue to right now.