BuddyPress - Youzify Change Time Stamp Format both for Comments and Posts:

bp_activity_time_since_newformat.php

<?php 
add_filter('bp_activity_time_since', 'bp_activity_time_since_newformat', 10, 2);
function bp_activity_time_since_newformat( $time_since, &$actvitiy ) {
 // you can change the date format to "Y-m-d H:i:s"
$time_since = '<span class="time-since">' . date_i18n("Y-m-d H:i", strtotime( $actvitiy->date_recorded ) ) . '</span>';
return $time_since;
}
add_filter('bp_activity_comment_date_recorded', 'bp_activity_comment_date_recorded_newformat', 10, 2);
function bp_activity_comment_date_recorded_newformat( $date_recorded ) {
  global $activities_template;
   // you can change the date format to "Y-m-d H:i:s"
  $date_recorded = '<span class="time-since">' . date_i18n("F j, Y", strtotime(
  $activities_template->activity->current_comment->date_recorded ) ) . '</span>';
  return $date_recorded;
}