NSString与NSDate互相转换及NSDate比较

由NSDate
转换为NSString:

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSString *strDate = [dateFormatter stringFromDate:[NSDate date]];
NSLog(@"%@", strDate);

由NSString
转换为NSDate:

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSDate *date = [dateFormatter dateFromString:@"2010-08-04 16:01:03"];
NSLog(@"%@", date);

日期之间比较可用以下方法
-(BOOL)isEqualToDate:(NSDate *)otherDate;

与otherDate比较，相同返回YES

- (NSDate*)earlierDate:(NSDate *)anotherDate;

与anotherDate比较，返回较早的那个日期

- (NSDate*)laterDate:(NSDate *)anotherDate;

与anotherDate比较，返回较晚的那个日期