Two methods of working with preprocess variables

gistfile1.php

// Altering the render array
function YOUR_THEME_preprocess_field(&$variables) {
	//Giveaway node: field giveaway value
	//change to dollar format
	if ($variables['element']['#field_name'] == 'field_giveaway_value') {
		$dollar_value = money_format('$%i', $variables['element']['#items'][0]['safe_value']) . "\n";
		$variables['items'][0]['#markup'] = $dollar_value;
	}
	
}

// Adding new variables that we be sent to the template file
function YOUR_THEME_preprocess_node(&$variables) {
	
	if ($variables['type'] == 'giveaway') {
		$node = $variables['node'];
		
		//Format Giveaway end dates
		$timestamp = strtotime($node->field_giveaway_duration['und'][0]['value2']);		
		$variables['duration_endyear'] = format_date($timestamp, 'custom', 'Y');
		$variables['duration_endmonth'] = format_date($timestamp, 'custom', 'n');
		$variables['duration_endday'] = format_date($timestamp, 'custom', 'j');
		
	}
}

These functions reside in your themes template.php file.