need help to show in date and time

[mhonn]

i have a problem when convert timestamp to readable date format.......
i used $grid formatter:date but the response is NAN/NAN/NAN

here my query:

$grid->setColProperty("tstart", array("label"=>"tstart","formatter"=>"date", "editrules"=>array("required"=>true)));
 
the response in tstart column is NaN/NaN/NaN

any idea to change timestamp format to readable date format in $grid
thanks alot