如何比较两个日期？

我有三个日期：（1）上一个日期（2）当前日期（3）下一个日期， 我想检查 currentDate 是否晚于前一个日期且早于 nextDate。 我怎么做？

 NSDateFormatter *df= [[NSDateFormatter alloc] init];

[df setDateFormat:@"yyyy-MM-dd"];

NSDate *dt1 = [[NSDate alloc] init];

NSDate *dt2 = [[NSDate alloc] init];

dt1=[df dateFromString:@"2011-02-25"];

dt2=[df dateFromString:@"2011-03-25"];

NSComparisonResult result = [dt1 compare:dt2];

switch (result)

{

     case NSOrderedAscending: NSLog(@"%@ is greater than %@", dt2, dt1); break;

     case NSOrderedDescending: NSLog(@"%@ is less %@", dt2, dt1); break;

     case NSOrderedSame: NSLog(@"%@ is equal to %@", dt2, dt1); break;

     default: NSLog(@"erorr dates %@, %@", dt2, dt1); break;
}