I warned you that using dashes would cause problems because php recognizes it as European format: D-M-Y. That is why your final example wouldn't work.
You shouldn't need to reformat it in the bindColumn(). The webassist function should do that automatically if you use "t" data type. I think this would work:
$UpdateQuery->bindColumn("eventdate", "t", "".(isset($_POST["eventdate"])?$_POST["eventdate"]:"") ."", "WA_DEFAULT");