This hack will add the following to your locales page:
Calculate Time Difference: [Without Minute Difference] [With Minute Difference]

Minute difference is just if the minutes between your time and the servers time have a difference greater than 30 then take that into account.

To apply the hack find line 165 in /blogs/inc/VIEW/settings/_set_regional.form.php
And you should see:

	$Form->begin_fieldset( T_('Regional settings') );
	$Form->text_input( 'newtime_difference', $Settings->get('time_difference'), 3, T_('Time difference'),
				array( 'note'=>sprintf( '['. T_('in hours'). '] '. T_('If you\'re not on the timezone of your server. Current server time is: %s.'), date_i18n( locale_timefmt(), $servertimenow ) ), 'maxlength'=> 3, 'required'=>true ) );

If you do, then insert this after it:

	// Start of balupton's hack
	$d = split(':',date_i18n( 'H:i', $servertimenow ));
	
	?>
	<fieldset style="padding-top:0px; margin-top:0px;">
	<div class="label"><label>Calculate Time Difference:</label></div>
	<div class="input" style="padding-top:0px; margin-top:0px;">
	<input type="button" onclick="calc_TimeDifference(false);" value="Without Minute Difference" />
	<input type="button" onclick="calc_TimeDifference(true);" value="With Minute Difference" />
	</div>
	</fieldset>
	<script type="text/javascript">
	
	function calc_TimeDifference(min_dif) {
	
	if ( ntd == null ) {
		var ntd = document.getElementById('newtime_difference');
		
		var server_hours 	= <?php echo $d[0]; ?>;
		var server_mins 	= <?php echo $d[1]; ?>;
	
		var user_date 		= new Date();
		var user_hours 		= user_date.getHours();
		var user_mins 		= user_date.getMinutes();
	}
	
	//alert('Server Hours: '+server_hours+'\r\nUser Time: '+user_hours);
	
	var result = 0;
	if( user_hours < server_hours ) {
	
		// We times by -1, because there is a negative difference
		result = server_hours - user_hours;
		// We do the following because if the time difference is greater than 12 then we are in a different day
		if( result > 12 ) result = -1*(24-result);
		result *= -1; // Just simplifies things.
		
	} else if( user_hours > server_hours ) {
	
		// We times by -1, because there is a negative difference
		result = user_hours - server_hours;
		// We do the following because if the time difference is greater than 12 then we are in a different day
		if( result > 12 ) result = -1*(24-result);
			
	}
	
	//alert('Time Difference: '+result);
		
	if( min_dif && ! ( user_mins < server_mins+30 && user_mins > server_mins-30 ) ) {
			if( user_mins < server_mins )
				result--;
			else
				result++;
			//alert('Time Difference: '+result+'\r\nMinute Difference Applied');
	}
	
	//alert('Server Time: '+server_hours+'\r\nUser Time: '+user_hours+'\r\nTime Difference: '+result);
		
	// Apply the calculated time difference
	ntd.value = result;
	
	}
		
	</script>
	<?php
	// End of balupton's hack

Then your all done.

Probably a smarter and easier idea would of been using the time zones e.g. gmt+8 etc, but this way works.