NSDate关闭一天？

Question

I'm attempting to compare two dates together to decide how much time is between them. For some reason I keep getting it all by one day off. I'm not too sure what is causing it, but I believe it to be the time zones? Do I have to specifically set that?

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];

[formatter setDateFormat:@"MM/dd/yy"];

newAssignment.dueDate = [formatter stringFromDate:self.dueDatePicker.date];

NSDate *today = [NSDate date];

//Difference between dates test
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

int unitFlags = NSDayCalendarUnit;

NSDateComponents *comps = [gregorian components:unitFlags fromDate:today toDate:self.dueDatePicker.date options:0];
newAssignment.daysUntilAssignmentDueInt = [comps day];

//Test to compare dates
NSLog(@"\n\ntoday's date: %@\ndate picked: %@\ndue in: %d\n", today, self.dueDatePicker.date, newAssignment.daysUntilAssignmentDueInt);

My NSLog outputs:

today's date: 2013-11-03 03:17:26 +0000
date picked: 2013-11-05 04:17:12 +0000
due in: 1

I've gone so much as to think today's Daylight Savings change would be changing all of this. But I'm just not sure anymore.

EDIT: I've read a few other posts here on StackOverflow, but I can't seem to see anyone who is experiencing what I am.

Answer 1

I've used nearly identical code to what you posted and I get correct results.

My code is a class method that relies on a global gregorian calendar that is set up in advance by the class's +initialize method, but the idea is the same. Here's my method:

+ (NSInteger) daysBetweenStartDate: (NSDate*) startDate andEndDate: (NSDate*) endDate;
{
    NSDateComponents* dateComponents = [gregorian components: NSDayCalendarUnit fromDate: startDate toDate: endDate options: 0]; 
    return dateComponents.day;
}