IOS-NSDate之今天，昨天，这周，这个月，上个月

NSDate *today = [NSDate dateWithString:@"2009-12-11 00:00:00 +0000"];
NSDate *yesterday = [NSDate dateWithString:@"2009-12-10 00:00:00 +0000"];
NSDate *thisWeek = [NSDate dateWithString:@"2009-12-06 00:00:00 +0000"];
NSDate *lastWeek = [NSDate dateWithString:@"2009-11-30 00:00:00 +0000"];
NSDate *thisMonth = [NSDate dateWithString:@"2009-12-01 00:00:00 +0000"];
NSDate *lastMonth = [NSDate dateWithString:@"2009-11-01 00:00:00 +0000"];
要求出上面的时间

NSCalendar *cal = [NSCalendar currentCalendar];
NSDateComponents *components = [cal components:( NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit ) fromDate:[[NSDate alloc] init]];

[components setHour:-[components hour]];
[components setMinute:-[components minute]];
[components setSecond:-[components second]];
NSDate *today = [cal dateByAddingComponents:components toDate:[[NSDate alloc] init] options:0]; //This variable should now be pointing at a date object that is the start of today (midnight);

[components setHour:-24];
[components setMinute:0];
[components setSecond:0];
NSDate *yesterday = [cal dateByAddingComponents:components toDate: today options:0];

components = [cal components:NSWeekdayCalendarUnit | NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit fromDate:[[NSDate alloc] init]];

[components setDay:([components day] - ([components weekday] - 1))];
NSDate *thisWeek  = [cal dateFromComponents:components];

[components setDay:([components day] - 7)];
NSDate *lastWeek  = [cal dateFromComponents:components];

[components setDay:([components day] - ([components day] -1))];
NSDate *thisMonth = [cal dateFromComponents:components];

[components setMonth:([components month] - 1)];
NSDate *lastMonth = [cal dateFromComponents:components];

NSLog(@"today=%@",today);
NSLog(@"yesterday=%@",yesterday);
NSLog(@"thisWeek=%@",thisWeek);
NSLog(@"lastWeek=%@",lastWeek);
NSLog(@"thisMonth=%@",thisMonth);
NSLog(@"lastMonth=%@",lastMonth);