Sunday, January 2, 2011

iOS 101: How to convert a String to a NSDate

During my vacations, I took some time to play with iOS development. I have been struggling with many small issues... This is the price to pay when learning a new technology, and this is part of the fun of doing it. I will try to document some of these issues in articles...  Let's start with a very common story : working with date.

Objective-C and iOS SDK provide a class to help formatting date (marshaling and unmarshaling), this class is NSDateFormatter. No surprise, the NSDateFormatter uses the Unicode Date Format Patterns.

A small example of date creating from a string:

    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc]init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd"];
    NSDate *date = [dateFormatter dateFromString:publicationDate ];
    [dateFormatter release];
     // use your date object

The date that I have to create from a sting looks like "2010-11-12". So I do not have any time information. When I do convert this string with the code above, the result is  "2010-11-11 23:00:00 +0000". As you can see the date is calculated from my current time zone, small reminder I am in France. So the "date" object itself is perfectly fine, but in my example I want to have the date independently of the time.

To be able to manage the date without any time/timezone information, I can force the timezone I want to use when using the  NSDateFormatter class. I just need to use the setTimeZone instance method.
The code looks like that now (see line#3):
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc]init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd"];
    [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
    NSDate *date = [dateFormatter dateFromString:publicationDate ];
    [dateFormatter release];
     // use your date object

Hope that helps!


abd said...

Thanks for the tip!

Zaki Shaheen said...

If you want to store these time in a database or send it over the is to use Unix timestamps. Here's a little snippet to get that:

+ (NSTimeInterval)getUTCFormateDate{

NSDateComponents *comps = [[NSCalendar currentCalendar]
components:NSDayCalendarUnit | NSYearCalendarUnit | NSMonthCalendarUnit
fromDate:[NSDate date]];
[comps setHour:0];
[comps setMinute:0];
[comps setSecond:[[NSTimeZone systemTimeZone] secondsFromGMT]];

return [[[NSCalendar currentCalendar] dateFromComponents:comps] timeIntervalSince1970];

Anonymous said...

Thanks for the tips this was driving me crazy !

Phil Ringsmuth said...

Thanks for this quick tip, it does exactly what I needed.

Phil Ringsmuth said...

Thanks for this little tip, it did exactly what I needed!