Using addTimeInterval in NSDate

Question

I am just learning Objective-C programming. I have built a small Window with an NSDatePicker and two buttons. I want the buttons to increase/decrease the date in the DatePicker by 1 day. Here is the code I put together:
@implementation CASDateIncrementor
- (IBAction)DownOneDay:(id)sender {
NSTimeInterval secondsPerDay = 24 * 60 * 60;
[DateToUse setDateValue:[NSDate addTimeInterval:secondsPerDay]];
It compiles with these warnings;
/Users/casdvm/Desktop/Date Mover/CASDateIncrementor.m:6: warning: 'NSDate' may not respond to '+addTimeInterval:'
/Users/casdvm/Desktop/Date Mover/CASDateIncrementor.m:6: warning: (Messages without a matching method signature will be assumed to return 'id' and accept '...' as arguments.)
The Console has this to say:
2008-03-19 07:02:51.473 Date Mover[2080:10b] * +[NSDate addTimeInterval:]: unrecognized selector sent to class 0xa0987940
The application indeed runs, but nothing happens when a button is pressed.
Alternatively, if I use this code:
NSTimeInterval secondsPerDay = 24 * 60 * 60;
[DateToUse setDateValue:[NSDate dateWithTimeIntervalSinceNow:secondsPerDay]];
It works as advertised, in that the date in the DatePicker is increased by a single day with the correct button press. Of course, it only works once, since now the date is not current.
I really did not think that this was going to be too terribly difficult; is there some syntactical reason that my addTimeInterval code is not working, or is the method dysfunctional? I do not understand the *Messages without a matching method signature* error and cannot locate advice on how to approach it.
Any advice or counsel is appreciated.

Answer 1

I think the problem you're having is that addTimeInterval functions on an actual NSDate variable by adding seconds to an existing date. But you're effectively applying it to the abstract NSDate type which is not instantiated. The dateWithTimeIntervalSinceNow works because it returns a value all on its own instead of adding or altering an existing value.
NSTimeInterval secondsPerDay = 24 * 60 * 60;
NSDate *today = [NSDate date];
NSDate *tomorrow;
tomorrow = [today addTimeInterval:secondsPerDay];
This works because today is an actual variable. What you're doing is essentially saying "int + 5" rather than "int x = 0; x+ 5;"
Hope that's clear. Also if you post your code bracketed by { code } markers (without the spaces) it will make you code more readable like mine above.
=Tod