NSDateFormatter *isoDateFormatter = [NSDateFormatter new];
[isoDateFormatter setDateFormat:@"yyyy-MM-dd"];
[isoDateFormatter setTimeZone:[NSTimeZone systemTimeZone]];
[isoDateFormatter setFormatterBehavior:NSDateFormatterBehaviorDefault];
NSString * str = [isoDateFormatter dateFromString:@"2008-12-31T12:00:00"]
在4.0 IOS上面的代码即str是nill,在3.0 IOS上这个工作正好是2008-12-31.如何克服这个问题？