CoreData predicate by date - nsdate

Question

I have a coreData model already setup and data added to it. I want to search for all items from the last 30 days, and then add together a total number of units.
Here's what I got :-
- (void) calculateThirtyDayValues {
NSDate *endDate = [NSDate date];
NSTimeInterval timeSinceRefDate = [NSDate timeIntervalSinceReferenceDate];
NSTimeInterval lastThirtyDays = timeSinceRefDate- 2592000;
NSDate *startDate = [NSDate dateWithTimeIntervalSinceReferenceDate:lastThirtyDays];
NSPredicate *predicate = [NSPredicate predicateWithFormat: #"(date >= %#) AND (date <= %#)", startDate, endDate];
}
Basically I have create an NSDate object set todays date, and then created another NSDate object set to 30 days before today. Then trying to predicate all objects from the start date until the present date.
I am getting results returned, but they don't seem to be really matching up with what the totals should be for the past 30 days. It appears to just be returning everything!
Any ideas?

Answer 1

Your predicate seems to be OK, but your date code is incorrect, for a couple of reasons.
[NSDate date] returns the current date, with sub-millisecond precision. So if you create the predicate at 4:15:37 PM local time, this predicate would not find any objects with a date of 5:37:42 PM local time. If you want down-to-the-second precision like that, then you're probably OK. But if you want granularity to a different calendar unit (such as by day), then you need to do more work.
Not every day has 86,400 seconds in it. Thus your attempt to subtract (30*86400) is subtly wrong. You should be letting the calendar object do the math for you:
NSDate *endDate = ...;
NSDateComponents *diff = [[NSDateComponents alloc] init];
[diff setDay:-30];
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDate *startDate = [calendar dateByAddingComponents:diff toDate:endDate options:0];