<?php

$data = '[{"Time": "2012-11-27 16:10:35", "NumPlayers": "1"}, {"Time": "2012-11-27 16:24:55", "NumPlayers": "1"}]';

// Make sure date is parsed as UTC
date_default_timezone_set("UTC");

// Convert items into the desired format
function mapper($item) {
    return array(strtotime($item->Time) * 1000, $item->NumPlayers);
}

echo json_encode(array_map("mapper", json_decode($data)));
?>

[[1354032635000,"1"],[1354033495000,"1"]]