<?php

$data1 = '2018-01-09 16:14:01';
$data2 = '2018-01-09 17:30:04';

$unix_data1 = strtotime($data1);
$unix_data2 = strtotime($data2);


$nHoras   = ($unix_data2 - $unix_data1) / (60 * 60);
$nMinutos = (($unix_data2 - $unix_data1) % (60 * 60)) / 60;

printf('%02d:%02d', $nHoras, $nMinutos);