Adapted from the Date and Time Programming Guide:
// Right now, you can remove the seconds into the day if you want
NSDate *today = [NSDate date];
// All intervals taken from Google
NSDate *yesterday = [today dateByAddingTimeInterval: -86400.0];
NSDate *thisWeek = [today dateByAddingTimeInterval: -604800.0];
NSDate *lastWeek = [today dateByAddingTimeInterval: -1209600.0];
// To get the correct number of seconds in each month use NSCalendar
NSDate *thisMonth = [today dateByAddingTimeInterval: -2629743.83];
NSDate *lastMonth = [today dateByAddingTimeInterval: -5259487.66];
If you want the correct exact number of days depending on the month, you should use an NSCalendar
.