小结：
1）日期格式化:NSDateFormatter,NSLocale, "yyyy-MM-dd HH:mm:ss"
2）字符转换为日期；NSDate *mydate=[df dateFromString:mydatestr];
3）日期比较：compare,NSOrderedSame,NSOrderedAscending,NSOrderedDescending
NSDate *dateDay=[[NSDate alloc] init];
NSDateFormatter *df=[[NSDateFormatter alloc] init];
[df setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSLocale *locale=[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];
[df setLocale:locale];
NSLog(@"dateDay =%@",dateDay);
NSString *mydatestr=@"2012-09-10 18:20:00";
NSDate *mydate=[df dateFromString:mydatestr];
NSLog(@"mydate =%@",mydate);
switch ([dateDay compare:mydate]) {
case NSOrderedSame:
NSLog(@"same");
break;
case NSOrderedAscending:
NSLog(@"dateday is earlier than mydate");
break;
case NSOrderedDescending:
NSLog(@"dateday is later than mydate");
break;
default:
NSLog(@"Bad date");
break;
}