How can i convert a datetime string like this: 2012-09-03 22:00 US Estern to UTC timezone?
You should look at the DateTime object and its related functions in the documentation.
If your input date is already in a string format, DateTime::createFromFormat() will help you create an epoch-type integer date the object can work with. After that, it's just getTimezone() and setTimezone().
You could convert your timestamps from DB to epoch time with strtotime and wrap them in a special CSS class :
<span class="timestamp"><?php echo strtotime('2012-09-03 22:00'); ?></span>
And then with a bit of a Javascript (jQuery example), dynamically transform the unix epoch into a locale datetime :
$('.timestamp').each(function(){
var seconds = $(this).html();
var d = new Date(1000 * parseInt(seconds));
var t = (d.getMonth()<9 ? '0':'') + parseInt(d.getMonth()+1) + '/';
t+= (d.getDate()<10 ? '0' : '') + parseInt(d.getDate()) + '/';
t+= parseInt((d.getYear()<1000 ? d.getYear()+1900 : d.getYear())) + ' ';
t+= (d.getHours()<10 ? '0' : '') + d.getHours() + ':' + (d.getMinutes()<10 ? '0' : '') + d.getMinutes();
$(this).html(t);
});
This will output dates in the format mm/dd/yyyy hh:ii, relative to browser timezone.