is my date difference calculation correct?

Question

-(NSString *)calculateTimeDifference:(NSString *)timeStamp{
// get the current date
NSDate *date = [NSDate date];
NSDateFormatter *dateFormat = [[NSDateFormatter alloc]init];
[dateFormat setDateFormat:@"yyyy-MM-dd HH:mm a"];

// convert it to a string
NSString *dateStringDev = [dateFormat stringFromDate:date];
DebugLog(@"system timeStamp is %@",dateStringDev );

//convert string from webService to NSDate

NSString *dateString = [timeStamp substringToIndex:[timeStamp length] - 5];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];

[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm a"];
NSTimeInterval timeInterval = 14;
[dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:timeInterval]];
[dateFormatter setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"GMT-7:58"]];

NSDate *dateFromString = [[NSDate alloc] init];

dateFromString = [dateFormatter dateFromString:dateString];
DebugLog(@"webserver timeStamp is %@",dateFromString);

//calculate Time Difference
NSTimeInterval timeDifference = [date timeIntervalSinceDate:dateFromString];

int iminutes = timeDifference / 60;
int ihours = iminutes / 60;
int idays = iminutes / 1440;


DebugLog(@"%d, %d, %d", idays, ihours, iminutes);
if (iminutes > 60) {
    while (iminutes>60) {
        iminutes = iminutes/60;
        ihours = ihours+ iminutes% 60;
    }

}
if (ihours > 24) {
    while (ihours>24) {
        ihours = ihours/24;
        idays = idays+ ihours% 24;
    }

}

if (idays == 0 && ihours == 0){
    timeStamp = [NSString stringWithFormat:@"%d minutes ago",iminutes];

}
else
    if ((idays == 0 ) && (ihours != 0) ){

        timeStamp = [NSString stringWithFormat:@"%d hours %d minutes ago",ihours,iminutes];
    }

    else if (idays != 0 && ihours != 0){
        switch (idays) {
            case 1:{
                NSArray *arr = [timeStamp componentsSeparatedByString: @" "];
                NSString *strSubStringDigNum = [arr objectAtIndex:1];
                timeStamp = [NSString stringWithFormat:@"yesterday %@",strSubStringDigNum];

            }
                break;
            default:
            { timeStamp= [timeStamp substringToIndex:[timeStamp length] - 5];
                NSArray *arr = [timeStamp componentsSeparatedByString: @" "];
                NSString *strSubstrDate = [arr objectAtIndex:0];
                NSString *strSubstrHours = [arr objectAtIndex:1];
                ihours = strSubstrHours.integerValue;
                NSArray *arrMin= [strSubstrHours componentsSeparatedByString: @":"];
                NSString *strSubstrMin = [arrMin objectAtIndex:1];
                iminutes = strSubstrMin.integerValue;
                if (ihours > 12) {
                    ihours = ihours - 12;
                    timeStamp = [NSString stringWithFormat:@"%@ %d:%d PM",strSubstrDate,ihours,iminutes ];
                    break;
                }
                else
                    timeStamp = [NSString stringWithFormat:@"%@ %d:%d AM",strSubstrDate,ihours,iminutes];
                break;

            }
        }

    }

return timeStamp;

}

timeStamp is my webservice Time as it is in 2012-11-29 06:11:16.0 format i am chopping 5 from last to calculate time difference and i tried with all STD GMT format non gave 0 min difference for present time with GMT -7.58 i got 0 diff , please suggest how to get device Abbreviations and to which format i should convert the webservice string to get proper time difference ..

any help much appreciated .

Answer 1

following are the code changes done in order to get proper time difference in LocalTime Zone //Get Local TimeZone NSTimeZone* localTimeZone = [NSTimeZone localTimeZone]; NSString* localAbbreviation = [localTimeZone abbreviation]; // To transform it back from the localAbbreviation, is just a matter to re-create the timeZone:

 NSTimeZone* timeZoneFromAbbreviation = [NSTimeZone timeZoneWithAbbreviation:localAbbreviation];
 NSString* timeZoneIdentifier = timeZoneFromAbbreviation.name;
 DebugLog(@"Identifier: %@", timeZoneIdentifier);

 // get the current date in LocalTimeZone
 NSDate *date = [NSDate date];
 DebugLog(@"system date in GMT %@",date );
 NSTimeZone *tz = [NSTimeZone defaultTimeZone];
 NSInteger seconds = [tz secondsFromGMTForDate: date];
 date = [NSDate dateWithTimeInterval: seconds sinceDate: date];
 DebugLog(@"system date  in Local time zone %@",date );


 //convert string from webService to NSDate
 NSDateFormatter *dateFormat2 = [[NSDateFormatter alloc] init];
 [dateFormat2 setTimeZone:[NSTimeZone timeZoneWithAbbreviation:timeZoneIdentifier]];
 [dateFormat2 setDateFormat:@"yyyy-MM-dd HH:mm:ss.0"];
 NSDate *dateFromWebServiceString = [[NSDate alloc] init];
 dateFromWebServiceString = [dateFormat2 dateFromString:timeStamp];
 DebugLog(@"webserver time in date as is %@",dateFromWebServiceString);



 //Calculate difference in WebService Time and System Time

 NSTimeInterval timeDifference = [date timeIntervalSinceDate:dateFromWebServiceString];
 DebugLog(@"%f",timeDifference);*/


if(timeDifference < 0)  // Server time is 2 minutes ahead of device time.   So as soon as post is done it gives -2 minutes ago instead of 0 minutes ago
{
    timeDifference = 0;
}

int iminutes = timeDifference / 60;
int ihours = iminutes / 60;
int idays = iminutes / 1440;

iminutes = iminutes - ihours * 60 ;
ihours = ihours - idays *24 ;// this ives correct no of days than the looping in while several times


DebugLog(@"%d, %d, %d", idays, ihours, iminutes);