Date handling can't cope with UTC higher than 12 eg UTC +14 (Pacific/Kiritimati)

All date handling in moodlelib.php assumes a timezone > 13

These functions are ones I found that need fixing:
usergetdate(), date_format_string(), make_timestamp(), usertimezone(), usertime()

1.9 codebase example from reporter (but this is still an issue in 2.x)

$usertime = userdate(gmmktime(), get_string('strftimedaydatetime'), "Pacific/Kiritimati" );

echo $usertime yields: "09/10 22:48"

When I use the below code, the time is correctly displayed:

$userDateTime = new DateTime(null, new DateTimeZone("Pacific/Kiritimati"));
$usertime = $userDateTime->format('m/d H:i');

echo $usertime yields: "09/10 16:48"

More discussion on this here:
https://moodle.org/mod/forum/discuss.php?d=237545