how to change nsdate in nsdictionary - iphone

Question

I am trying to increment my NSDate. I have an NSDictionary, I am trying to increment my NSDate content in it. Say my NSDate is 2011-07-11, I want to increment the NSDate content in it.
got
{
ConditionDatenew = "2011-07-21 13:31:46 +0000";
Yesterday = "2011-07-20 13:31:46 +0000";
city = #;
condition = "Isolated Thunderstorms";
country = #;
"day_of_week" = Sun;
high = 86;
icon = "chance_of_storm.gif";
low = 68;
state = #;
}
I just want to get the date as 2011-07-22 in my ConditionDatenew in the next dictionary loop. How can I get it?

Answer 1

Create a new date object, and replace the old value in the dictionary. Dates are immutable.
NSDate *newDate = [NSDate date];
[dictionary setObject:newDate forKey:#"ConditionDatenew"];
[NSDate date] will set this to "now."
The easiest way to add a day is to add 24 hours. This works as long as DST is never an issue, and by "increment a day" you mean "increment by 24 hours." If you work exclusively in UTC, then that's fine.
NSDate *newDate = [oldDate dateByAddingTimeInterval:24*60*60];
If DST is an issue (and it usually is), and by "increment a day" you mean "increment by a Gregorian calendar day" then you need to use date components to make sure you add 23, 24 or 25 hours as appropriate.
NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
[dateComponents setYear:2011];
[dateComponents setMonth:11];
[dateComponents setDay:6];
[dateComponents setTimeZone:[NSTimeZone timeZoneWithName:#"America/New_York"]];
NSDate *date = [calendar dateFromComponents:dateComponents];
NSDateComponents *addComponents = [[NSDateComponents alloc] init];
[addComponents setDay:1];
[addComponents setTimeZone:[NSTimeZone localTimeZone]];
NSDate *newDate = [calendar dateByAddingComponents:addComponents toDate:date options:0];
NSLog(#"oldDate=%#", [date descriptionWithLocale:[NSLocale currentLocale]]);
NSLog(#"+24=%#", [[date dateByAddingTimeInterval:24*60*60] descriptionWithLocale:[NSLocale currentLocale]]);
NSLog(#"newDate=%#", [newDate descriptionWithLocale:[NSLocale currentLocale]]);
[calendar release];
[dateComponents release];
[addComponents release];
Output:
oldDate=Sunday, November 6, 2011 12:00:00 AM Eastern Daylight Time
+24=Sunday, November 6, 2011 11:00:00 PM Eastern Standard Time
newDate=Monday, November 7, 2011 12:00:00 AM Eastern Standard Time
If you don't care about the time, and have control over what you set it to, one trick is to set the time to noon. That way adding 24 hours will always fall on the correct day. I don't generally recommend this because it fails badly if you ever forget and create a date with a time of midnight. It's easier to put all of your day-incrementing code in one place and fix the DST problem one time than to make sure all of your date creation code is always right. So I recommend getting used to date components.

Answer 2

Try this:
NSDate *newDate = [NSDate date];
[dictionary setValue:newDate forKey:#"ConditionDatenew"];