Format of date when retrieving lastModified - objective-c

Question

I am comparing the date when a file was last modified for two files, one local and one on Amazon S3 server. I am using the AWS IOS SDK framework and can successfully request and receive response from the S3 server but I have trouble understanding the format of the returned s3 date.
On my local machine the date format for lastModified is "2011-07-21 18:43:15 -0400" while for the file residing on the S3 server it is "2011-10-15T16:25:49.000Z".
My local info is obtained using:
NSFileManager *fm = [NSFileManager defaultManager];
NSDictionary *attr = [fm attributesOfItemAtPath:filePath error:nil];
NSDate *localDate = [attr objectForKey:NSFileModificationDate];
while my S3 info is obtained using
for (S3ObjectSummary *object in [listObjectsResult objectSummaries]) {
NSDate *s3date = [object lastModified];
}
Does anyone know if I can convert the date for the S3 file to a format that I can use to compare these two dates using:
NSTimeInterval deltaSeconds = [s3Date timeIntervalSinceDate: localDate];
or am I doing something wrong here? Right now my program crashes with
[NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance 0x200351360.
probably because the s3 date format is not in proper format. I am quite new to using the AWS S3 SDK so all help is greatly appreciated. If anyone also knows of some good tutorials for this framework (apart from the demo code that comes with it), that would be great. Cheers, Trond

Answer 1

It looks to me as [object lastModified] simply returns a NSString and not an NSDate object, as stated in the documentation.
NSDateFormatter can be used in this case to create a NSDate object from the string:
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"yyyy'-'MM'-'dd'T'HH':'mm':'ss'+'mm':'ss.SSS'Z'"];
NSDate *s3date = [dateFormatter dateFromString:[object lastModified]];
[dateFormatter release];
The Date Formatting guide has lots of handy examples. You may need to tweak the format string slightly as i have not tested it.

Answer 2

Had some trouble with the given answer - seems that there is some sort of duplicated 'ss'+'mm' stuff in there that prevents the string from correctly translating. Additionally, because the time zone in the string is given as UTC, you have to set the time zone for the formatter. Here is the code I used (that worked for me), I turned it into a category, and default to using the previously provided string in the event the one that works for me doesn't work for you:
#interface S3ObjectSummary (lastModifiedDate)
- (NSDate *)lastModifiedDate;
#end
#implementation S3ObjectSummary (lastModifiedDate)
- (NSDate *)lastModifiedDate
{
NSString *modifiedString = [self lastModified];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init]; // Note, this is ARC, add autorelease if you aren't ARC
[dateFormatter setTimeZone:[NSTimeZone timeZoneWithName:#"UTC"]];
[dateFormatter setDateFormat:#"yyyy'-'MM'-'dd'T'HH':'mm':'ss.SSS'Z'"];
NSDate *modifiedDate = [dateFormatter dateFromString:modifiedString];
if(!modifiedDate) // Try other format in case of fail http://stackoverflow.com/a/7799607/285694
{
[dateFormatter setDateFormat:#"yyyy'-'MM'-'dd'T'HH':'mm':'ss'+'mm':'ss.SSS'Z'"];
modifiedDate = [dateFormatter dateFromString:modifiedString];
}
return modifiedDate;
}
#end

Answer 3

Looks like this also works:
NSDate *summaryDate = [NSDate dateWithISO8061Format:summary.lastModified];