Comparing two NSDate - ios

Question

I have a UIViewController with 2 buttons: today and yesterday. Every time the client clicked on today it needs to set the property _date for today and the same with yesterday.
The method which I set the property value is the following:
-(IBAction)dateButtonTouched:(id)sender
{
if(![sender isSelected])
{
NSCalendar *cal = [NSCalendar currentCalendar];
NSDateComponents *components = [cal components:( NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit ) fromDate:[[NSDate alloc] init]];
[components setMinute:-[components minute]];
[components setSecond:-[components second]];
if([sender tag] == YESTERDAY)
{
[components setHour:-24];
_date= [cal dateByAddingComponents:components toDate: [NSDate date] options:0]; //YESTERDAY
}
else
{
NSDate *today = [NSDate date];
[components setHour:0];
_date= [cal dateByAddingComponents:components toDate: today options:0]; //TODAY
}
NSLog(#"user selected date %#",_date);
}
}
in the following method I'm trying to check if _date is today or yesterday:
-(void) viewWillLayoutSubviews
{
NSDateFormatter *dateFormat = [NSDateFormatter new];
[dateFormat setDateFormat:#"MM-dd-YY"];
self.dateLabel.text = [dateFormat stringFromDate:_date];
NSCalendar *cal = [NSCalendar currentCalendar];
NSDateComponents *components = [cal components:( NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit ) fromDate:[[NSDate alloc] init]];
[components setMinute:-[components minute]];
[components setSecond:-[components second]];
[components setHour:0];
NSDate *today = [cal dateByAddingComponents:components toDate: [NSDate date] options:0]; //TODAY
[components setHour:-24];
NSDate *yesterday = [cal dateByAddingComponents:components toDate: [NSDate date] options:0]; //YESTERDAY
if([_date isEqualToDate:yesterday])
{
//do something
}
else if( [_date isEqualToDate:today])
{
//do something
}
}
The _date is never today or yesterday although if I debug it, print it or compare the description is the same.

Answer 1

An NSDate instance represents a specific moment in time. It is much more specific than "yesterday" or "today". Instead of trying to reset the hours, minutes, and seconds via date components, Maybe you should compare the components you're interested in: day, month, and year. Or you can use a calendar to compare at the component level:
NSDate *today = [NSDate date];
NSDate *someDay; // the date to compare
NSCalendar *cal = [NSCalendar currentCalendar];
NSDateComponents *difference = [cal components:NSDayCalendarUnit fromDate:today toDate:someDay options:nil];
if (difference.day == -1) {
// yesterday
} else if (difference.day == 0) {
// today
}
Without knowing exactly what you want to do, it's harder to get more specific.

Answer 2

An NSDate object contains both date and time, encoded as a value that is essentially the number of microseconds since Jan 1 2000.
When you do two successive [NSDate date] operations you will get two different values, if not because the time has actually changed that much between the two then because most computer clock implementations assure that you get distinct values with each access.
NSDate's isEqualToDate function compares all of the bits of the two NSDate objects to see if they are exactly equal. This means that if the two are even a microsecond apart they will not appear to be equal.
If you want to see if two NSDate values are the same day you should either use NSDateFormatter to format them into strings and compare the strings, or use one of the NSDateComponent functions to calculate the number of days between them (several ways to do this).