NSDate - Change Time Component - iphone

Question

This is probably a silly question but I can't seem to find an answer.
I am using a date picker to allow a user to enter a date and it is also setting the time to the current time (I init the picker with [NSDate date]).
Is there a way to change the time component of the date? I am later calculating time intervals which returns seconds and I am getting some odd behavior because of the time.
This code shows the date entry:
#implementation AddEventViewController
#synthesize backgroundColor, nameField, dateField, datePicker, eventName, eventDate;
-(void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
self.view.backgroundColor = self.backgroundColor;
}
- (void)viewDidLoad
{
[super viewDidLoad];
dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateStyle:NSDateFormatterMediumStyle];
[dateFormatter setTimeStyle:NSDateFormatterNoStyle];
CGSize size = self.view.bounds.size;
CGRect frame = CGRectMake(0, size.height - 216, 320, 216);
datePicker = [[UIDatePicker alloc] initWithFrame: frame];
datePicker.datePickerMode = UIDatePickerModeDate;
datePicker.hidden = YES;
datePicker.date = [NSDate date];
[datePicker addTarget:self
action:#selector(changeDateInLabel:)
forControlEvents:UIControlEventValueChanged];
[self.view addSubview: datePicker];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
}
-(IBAction)doneButton
{
if ((nameField.text != nil))
{
self.eventName = nameField.text;
self.eventDate = datePicker.date;
[[NSNotificationCenter defaultCenter]postNotificationName:#"EventAddDone" object:nil];
}
else
{
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:#"Enter Event Name" message:nil delegate:nil cancelButtonTitle:#"Okay" otherButtonTitles:nil];
[alertView show];
[alertView release];
}
}
-(IBAction)cancelButton
{
[[NSNotificationCenter defaultCenter]postNotificationName:#"EventAddCancel" object:nil];
}
- (BOOL)textFieldShouldReturn:(UITextField *)textField
{
[textField resignFirstResponder];
return YES;
}
- (void)changeDateInLabel:(id)sender
{
dateField.text = [dateFormatter stringFromDate:[datePicker date]];
}
- (BOOL)textFieldShouldBeginEditing:(UITextField *)textField
{
if(textField == dateField)
{
dateField.placeholder = #" ";
dateField.text = [dateFormatter stringFromDate:[datePicker date]];
datePicker.hidden = NO;
[nameField resignFirstResponder];
return NO;
}
return YES;
}
- (void)dealloc
{
self.backgroundColor = nil;
self.nameField = nil;
self.dateField = nil;
self.datePicker = nil;
self.eventName = nil;
self.eventDate = nil;
[dateFormatter release];
[super dealloc];
}
#end
My use of the recorded date is similar to the following:
NSTimeInterval age = [enteredDate timeIntervalSinceNow];
//do some math on the age variable to get number of days since the event occurred.

Answer 1

Two options:
Option 1:
int fromDate = [[NSCalendar currentCalendar] ordinalityOfUnit:NSDayCalendarUnit
inUnit:NSEraCalendarUnit
forDate:enteredDate];
int toDate = [[NSCalendar currentCalendar] ordinalityOfUnit:NSDayCalendarUnit
inUnit:NSEraCalendarUnit
forDate:[NSDate date]];
int days = toDate - fromDate;
option 2:
NSDate *dateA, *dateB;
[[NSCalendar currentCalendar] rangeOfUnit:NSDayCalendarUnit
startDate:&dateA
interval:NULL
forDate:enteredDate];
[[NSCalendar currentCalendar] rangeOfUnit:NSDayCalendarUnit
startDate:&dateB
interval:NULL
forDate:[NSDate date]];
//From here, apply the above method using components:fromDate:toDate:
//with dateA and dateB, which are set to the beginning of the days on
//which they lie.

Answer 2

NSInteger days = [[[NSCalendar currentCalendar] components:NSDayCalendarUnit
fromDate:enteredDate
toDate:[NSDate date]
options:0] day];