UTCな時間（文字列）をNSDateなローカル時間に変換する

main

NSString *tmp = @"2013-09-03T08:29:16.575Z";
NSDate *date = [[tmp dateFromStringWithFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"] toLocalTime];

NSDate+カテゴリさま

- (NSDate *)toLocalTime
{
    NSTimeZone *tz = [NSTimeZone defaultTimeZone];
    NSInteger seconds = [tz secondsFromGMTForDate:self];
    return [NSDate dateWithTimeInterval:seconds sinceDate:self];
}

- (NSDate *)toGlobalTime
{
    NSTimeZone *tz = [NSTimeZone defaultTimeZone];
    NSInteger seconds = -[tz secondsFromGMTForDate:self];
    return [NSDate dateWithTimeInterval:seconds sinceDate:self];
}

NSString+カテゴリさま

- (NSDate *)dateFromStringWithFormat:(NSString *)format
{
    return [self dateFromStringWithTimeZone:[[[NSTimeZone alloc] initWithName:@"UTC"] autorelease] format:format];
}