NSString+Extensions.h

#import <Foundation/Foundation.h>

@interface NSString (Extensions)
+ (NSString *)timeAgoStringFromDate:(NSDate *)fromDate;
@end

NSString+Extensions.m

#import "NSString+Extensions.h"

#define FormatterString @"%i %@ ago"

@implementation NSString (Extensions)

+ (NSString *)timeAgoStringFromDate:(NSDate *)fromDate
{
    NSUInteger desiredComponents = NSYearCalendarUnit | NSMonthCalendarUnit | NSWeekCalendarUnit | NSHourCalendarUnit | NSDayCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;
    NSDateComponents *elapsedTime = [[NSCalendar currentCalendar] components:desiredComponents 
                                                                         fromDate:fromDate
                                                                           toDate:[NSDate date]
                                                                          options:0];
    NSString *timeAgo = nil;
    
    if ([elapsedTime year]) {
        timeAgo = [NSString stringWithFormat:FormatterString, [elapsedTime year],
                   ([elapsedTime year] == 1) ? @"year" : @"years"];
    }
    else if ([elapsedTime month]) {
        timeAgo = [NSString stringWithFormat:FormatterString, [elapsedTime month],
                   ([elapsedTime month] == 1) ? @"month" : @"months"];
    }
    else if ([elapsedTime week]) {
        timeAgo = [NSString stringWithFormat:FormatterString, [elapsedTime week],
                   ([elapsedTime week] == 1) ? @"week" : @"weeks"];
    }
    else if ([elapsedTime day]) {
        timeAgo = [NSString stringWithFormat:FormatterString, [elapsedTime day],
                   ([elapsedTime day] == 1) ? @"day" : @"days"];
    }
    else if ([elapsedTime hour]) {
        timeAgo = [NSString stringWithFormat:FormatterString, [elapsedTime hour],
                   ([elapsedTime hour] == 1) ? @"hour" : @"hours"];
    }
    else if ([elapsedTime minute]) {
        timeAgo = [NSString stringWithFormat:FormatterString, [elapsedTime minute],
                   ([elapsedTime minute] == 1) ? @"minute" : @"minutes"];
    }
    else {
        timeAgo = [NSString stringWithFormat:FormatterString, [elapsedTime second],
                   ([elapsedTime second] == 1) ? @"second" : @"seconds"];
    }

    
    return timeAgo;
}

@end

problems.m

// set the time zone of the date value used
[dateFormatter setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"GMT"]];

// To set the time zone of the calendar that we use to get the elapsed time, do this:
NSCalendar *cal = [NSCalendar currentCalendar];
[cal setTimeZone:[NSTimeZone localTimeZone]];
NSDateComponents *elapsedTime = [cal components:desiredComponents fromDate:fromDate toDate:[NSDate date] options:0];