Utility class for caching NSDateFormatters, as suggested by: http://developer.apple.com/library/ios/#documentation/cocoa/Conceptual/DataFormatting/Articles/dfDateFormatting10_4.html

UUDateFormatter.h

#import <Foundation/Foundation.h>

extern NSString * const kUURFC3339DateFormatter;

@interface UUDateFormatter : NSObject

+ (NSDate*) dateFromRfc3339String:(NSString*)string;
+ (NSDate*) dateFromString:(NSString*)string withFormat:(NSString*)format;

+ (NSString*) dateToRfc3339String:(NSDate*)date;
+ (NSString*) dateToString:(NSDate*)date withFormat:(NSString*)format;
   
@end

UUDateFormatter.m

#import "UUDateFormatter.h"

static NSMutableDictionary* theSharedDateFormatterCache = nil;

NSString * const kUURFC3339DateFormatter = @"yyyy-MM-dd'T'HH:mm:ssZZ";

@implementation UUDateFormatter 

+ (NSDate*) dateFromRfc3339String:(NSString*)string
{
    return [self dateFromString:string withFormat:kUURFC3339DateFormatter];
}

+ (NSDate*) dateFromString:(NSString*)string withFormat:(NSString*)format
{
    NSDateFormatter* df = [self getDateFormatter:format];
    return [df dateFromString:string];
}

+ (NSString*) dateToRfc3339String:(NSDate*)date
{
    return [self dateToString:date withFormat:kUURFC3339DateFormatter];
}

+ (NSString*) dateToString:(NSDate*)date withFormat:(NSString*)format
{
    NSDateFormatter* df = [self getDateFormatter:format];
    return [df stringFromDate:date];
}

+ (NSMutableDictionary*) sharedCache
{
    if (theSharedDateFormatterCache == nil)
    {
        theSharedDateFormatterCache = [[NSMutableDictionary alloc] init];
    }
    
    return theSharedDateFormatterCache;
}

+ (NSDateFormatter*) getDateFormatter:(NSString*)formatter
{
    NSMutableDictionary* cache = [self sharedCache];
    NSDateFormatter* df = [cache valueForKey:formatter];
    if (df == nil)
    {
        df = [[NSDateFormatter alloc] init];
        [df setDateFormat:formatter];
        [cache setValue:df forKey:formatter];
    }
    
    return df;
}

@end