$event_start = date(’Y-m-d H:i:s’, strtotime($event_start));

Change the am/pm select box to have literal values. So the selection for “AM” would provide the value “AM”. Selection for “PM” would give the value “PM”.

Now, when the form is submitted concatenate the parts together into a string that is in US English date format

$event_start = $start_month . '/' . $start_day . '/'. $start_year . ' ' . $event_start_time_hh . ':' . $event_start_time_mm . ' ' . $event_start_time_ampm;

Then pass that string thru strtotime() and into date() to format however you need it.

echo date('Y-m-d H:i:s', strtotime($event_start));

Here’s a full example

// assume these values are set by your form being submitted
$start_month = '04';
$start_day = '06';
$start_year = '2010';
$event_start_time_hh = '2';
$event_start_time_mm = '30';
$event_start_time_ampm = 'pm';

// concatenate the parts into US English formatted date
$event_start = $start_month . '/' . $start_day . '/'. $start_year . ' ' . $event_start_time_hh . ':' . $event_start_time_mm . ' ' . $event_start_time_ampm;

// output full MySQL 24 hour format
echo date('Y-m-d H:i:s', strtotime($event_start));

//prints "2010-04-06 14:30:00"

For instance, if I have variables that I string together to show the date & time as:

$event_start = $start_year.”-”.$start_month.”-”.$start_day.” “.$event_start_time_hh.”:”.$event_start_time_mm.”:00″;

On my form I also have a field for my variable: $event_start_time_ampm with value for AM being ‘00′ and PM is ‘12′.

I was trying to make an if else statement:

if ($_POST['event_start_time_ampm'] == ‘12′) {
$_POST['event_start_time_hh'] = $_POST['event_start_time_hh'] + 12;
} else {
$_POST['event_start_time_hh'];
}

But, it’s not working, it only formats the time as AM.