Here is how I'd do it:
function dateToTimestamp($date, $format, $timezone='Europe/Belgrade')
{
//returns an array containing day start and day end timestamps
$old_timezone=date_timezone_get();
date_default_timezone_set($timezone);
$date=strptime($date,$format);
$day_start=mktime(0,0,0,++$date['tm_mon'],++$date['tm_mday'],($date['tm_year']+1900));
$day_end=$day_start+(60*60*24);
date_default_timezone_set($old_timezone);
return array('day_start'=>$day_start, 'day_end'=>$day_end);
}
$timestamps=dateToTimestamp('15.02.1991.', '%d.%m.%Y.', 'Europe/London');
$day_start=$timestamps['day_start'];
This way, you let the function know what date format you are using and even specify the timezone.