title: NSDateFormatter dateFromString returns null
body: <p>I want to concatenate 2 <code>NSDates</code> via <code>NSDateComponents</code> and <code>[NSString stringWithFormat]</code> to <code>[dtFormatter dateFromString]</code>. the first one has a complete format consisting of <code>yyyy-MM-dd HH:mm:ss</code>. But The second one has just information about hour and minute. So I want the second date to have all other information from the first date excepting seconds (will be added automatically?). I  think everything works well until dtFormatter formats string to date. I already searched on stackoverflow but no solution could fix my issue: </p>
  
  <p><strong>my code</strong>:</p>
  
  <pre><code>//pDate has a complete date format
  NSDateComponents *dateComponentsComplete = [[NSCalendar currentCalendar]
  components:NSYearCalendarUnit |NSDayCalendarUnit | NSMonthCalendarUnit fromDate:self.pDate];
  //sunRise has the incomplete format
  NSDateComponents *dateComponentsIncompleteSunRise = [[NSCalendar currentCalendar] components:NSHourCalendarUnit | NSMinuteCalendarUnit fromDate:self.sunRise];
  
  NSInteger day = [dateComponentsComplete day];
  NSInteger month = [dateComponentsComplete month];
  NSInteger year = [dateComponentsComplete year];
  
  NSInteger hour = [dateComponentsIncompleteSunRise hour];
  NSInteger minute = [dateComponentsIncompleteSunRise minute];
  
  //concatenate  
  NSString *strSunRise = [NSString stringWithFormat:@"%d-%d-%d %d:%d", year, month, day, hour, minute];
  
  NSLog(@"strSunRise %@ ",  strSunRise);
  
  NSDateFormatter* dtFormatter = [[NSDateFormatter alloc] init];
  [dtFormatter setFormatterBehavior:NSDateFormatterBehavior10_4];
  [dtFormatter setTimeZone:[NSTimeZone systemTimeZone]];
  [dtFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
  
  NSDate* dateOutput = [dtFormatter dateFromString:strSunRise];
  NSLog(@"%@", [dateOutput class]);
  NSLog(@"dateOutput %@ ", [dtFormatter stringFromDate:dateOutput]);
  </code></pre>
  
  <p><strong>output:</strong></p>
  
  <pre><code>strSunRise 2013-7-25 5:16 
  (null)
  dateOutput (null)
  </code></pre>