在 NSUserDefaults 中保存日期（作为 NSString）不起作用

我正在尝试将此日期保存在 NSUSerDefaults 中：

2011-04-14T13:18:25+0000

但我不想要最后一位（+0000），所以我用这个方法把它砍掉了：

NSString *since = [last objectForKey:@"updated_time"];
NSString *cutOff = @"+0000"; 
NSRange range = [since rangeOfString:cutOff];
self.lastUpdated = [since substringToIndex:range.location];
[[NSUserDefaults standardUserDefaults] setObject:lastUpdated forKey:@"lastUpdated"];

正如您所看到的，这只是一个非常简单的操作，将上面的字符串截断为：

2011-04-14T13:18:25

但是，当我从 NSUserDefaults 加载此内容时，我得到以下信息：

2011-04-14 13:18:25 +0000

谁能告诉我为什么？

UPDATE:

我像往常一样在 .h 中声明了 lastUpdated：

@interface TwitModel
{
    NSString *lastUpdated;
}

@property (retain) NSString *lastUpdated;

in .m:

@synthesize lastUpdated;

当我想加载它时：

    if(self.lastUpdated == nil) {
        self.lastUpdated = [[NSUserDefaults standardUserDefaults] objectForKey:@"lastUpdated"];
    }

你需要打电话synchronize on NSUserDefaults为了让它正确保存数据：

[[NSUserDefaults standardUserDefaults] synchronize];

EDIT

这是我的工作代码：

// Output previous launch's stored value
NSLog(@"Value From Previous Launch: %@", [[NSUserDefaults standardUserDefaults] valueForKey:@"lastUpdated"]);

NSString *since = @"2011-04-14T13:18:25+0000";
NSLog(@"Since: %@", since);
NSString *cutOff = @"+0000"; 
NSRange range = [since rangeOfString:cutOff];
NSString* lastUpdated = [since substringToIndex:range.location];
[[NSUserDefaults standardUserDefaults] setObject:lastUpdated forKey:@"lastUpdated"];
[[NSUserDefaults standardUserDefaults] synchronize];

NSLog(@"Value: %@", [[NSUserDefaults standardUserDefaults] valueForKey:@"lastUpdated"]);

和输出：

2011-04-14 08:52:29.972 TestApp[2117:207] Value From Previous Launch: 2011-04-14T13:18:25
2011-04-14 08:52:29.973 TestApp[2117:207] Since: 2011-04-14T13:18:25+0000
2011-04-14 08:52:29.975 TestApp[2117:207] Value: 2011-04-14T13:18:25