title: __NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance
body: <p>I'm trying to calculate duration difference between two times. I get the durations from JSONArray. I use the below code </p>
  
  <pre><code>NSDate *starttime = [[NSDate alloc]init];
  NSDate *endtime = [[NSDate alloc]init];
  starttime = [currentRecord valueForKey:@"starttime"];
  endtime = [currentRecord valueForKey:@"endtime"];
  </code></pre>
  
  <p>I pass the above values to another method </p>
  
  <pre><code>+(NSString*)remaningTime:(NSDate*)startDate endDate:(NSDate*)endDate
  {
   NSDateComponents *components= [[NSDateComponents alloc]init];
   NSString *durationString;
  
   components = [[NSCalendar currentCalendar] components:       NSCalendarUnitDay/*|NSCalendarUnitHour|NSCalendarUnitMinute*/ fromDate: endDate toDate: startDate options: 0];
  }
  </code></pre>
  
  <p>but I get below exception</p>
  
  <pre><code>-[__NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance 0x78685f80
  
  2016-12-20 15:21:37.361 nexge[1515:183039] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance 0x78685f80'
  *** First throw call stack:
  (
      0   CoreFoundation                      0x03abc212 __exceptionPreprocess + 194
      1   libobjc.A.dylib                     0x041d3e66 objc_exception_throw + 52
      2   CoreFoundation                      0x03b353dc -[NSObject(NSObject) doesNotRecognizeSelector:] + 172
      3   CoreFoundation                      0x03a3b34c ___forwarding___ + 1052
      4   CoreFoundation                      0x03a3af0e _CF_forwarding_prep_0 + 14
      5   CoreFoundation                      0x03a9b419 -[__NSCFCalendar components:fromDate:toDate:options:] + 393
      6   CoreFoundation                      0x03a9b217 -[_NSCopyOnWriteCalendarWrapper components:fromDate:toDate:options:] + 103
      7   nexge                               0x0003c2c8 +[callRecords remaningTime:endDate:] + 248
      8   nexge                               0x0003c0f9 -[callRecords initWithCallAmount:duration:start:end:destination:] + 393
      9   nexge                               0x00036321 -[callRecordsDataController addRecordWithCallAmount:duration:start:end:destination:] + 257
      10  nexge                               0x00036d0d -[callRecordsDataController connectionDidFinishLoading:] + 1165
      11  CFNetwork                           0x00464850 ___ZL32_NSURLConnectionDidFinishLoadingP16_CFURLConnectionPKv_block_invoke + 55
      12  CFNetwork                           0x004603b8 __65-[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:]_block_invoke + 71
      13  CFNetwork                           0x00460369 -[NSURLConnectionInternalConnection invokeForDelegate:] + 142
      14  CFNetwork                           0x004602c7 -[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:] + 188
      15  CFNetwork                           0x004601f7 -[NSURLConnectionInternal _withActiveConnectionAndDelegate:] + 42
      16  CFNetwork                           0x00464814 _ZL32_NSURLConnectionDidFinishLoadingP16_CFURLConnectionPKv + 36
      17  CFNetwork                           0x00464769 ___ZN27URLConnectionClient_Classic26_delegate_didFinishLoadingEU13block_pointerFvvE_block_invoke + 96
      18  CFNetwork                           0x00685f81 ___ZN27URLConnectionClient_Classic18_withDelegateAsyncEPKcU13block_pointerFvP16_CFURLConnectionPK33CFURLConnectionClientCurrent_VMaxE_block_invoke_2 + 107
      19  libdispatch.dylib                   0x04cb0cc3 _dispatch_client_callout + 14
      20  libdispatch.dylib                   0x04c895f2 _dispatch_block_invoke_direct + 380
      21  libdispatch.dylib                   0x04c9497f ___dispatch_block_create_block_invoke + 20
      22  CFNetwork                           0x004600ce _ZN19RunloopBlockContext13_invoke_blockEPKvPv + 24
      23  CoreFoundation                      0x03a06ef9 CFArrayApplyFunction + 57
      24  CFNetwork                           0x0045ffc9 _ZN19RunloopBlockContext7performEv + 139
      25  CFNetwork                           0x005664a8 _ZThn16_N19RunloopBlockContext24multiplexerClientPerformEv + 20
      26  CFNetwork                           0x0045fe65 _ZN17MultiplexerSource7performEv + 319
      27  CFNetwork                           0x0045fc84 _ZN17MultiplexerSource8_performEPv + 62
      28  CoreFoundation                      0x03a5bcbf __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
      29  CoreFoundation                      0x03a3fdf7 __CFRunLoopDoSources0 + 519
      30  CoreFoundation                      0x03a3f284 __CFRunLoopRun + 1124
      31  CoreFoundation                      0x03a3ebab CFRunLoopRunSpecific + 395
      32  CoreFoundation                      0x03a3ea0b CFRunLoopRunInMode + 123
      33  GraphicsServices                    0x0575eb4c GSEventRunModal + 177
      34  GraphicsServices                    0x0575e9c7 GSEventRun + 80
      35  UIKit                               0x0225c32b UIApplicationMain + 148
      36  nexge                               0x0003702c main + 140
      37  libdyld.dylib                       0x04ceb799 start + 1
      38  ???                                 0x00000001 0x0 + 1
  )
  libc++abi.dylib: terminating with uncaught exception of type NSException
  </code></pre>