Unable to sync computer time to Arduino via USB

Question

I want to sync the Time from my pc to the arduino. I am using their Time library but it does not work.

How can I get the arduino to have the same time as on my computer ? I am currently using a mac.

Their documentations says :

On a unix system, you can set the time with the shell command:

TZ_adjust=-8; echo T$(($(date +%s)+6060$TZ_adjust)) > /dev/tty.usbserial-A8008pym

I tried on the terminal

>export TZ_adjust=-8; echo T$(($(date +%s)+6060$TZ_adjust)) > /dev/tty.usbmodemfd131 

and I get Permission denied.

What am I doing wrong? Is there a simpler way to sync the time on arduino with my computer?

The Code

#include <Time.h>  

#define TIME_MSG_LEN  11   // time sync to PC is HEADER followed by unix time_t as ten ascii digits
#define TIME_HEADER  'T'   // Header tag for serial time sync message
#define TIME_REQUEST  7    // ASCII bell character requests a time sync message 

void setup()  {
  Serial.begin(9600);
  setSyncProvider( requestSync);  //set function to call when sync required
  Serial.println("Waiting for sync message");
}

void loop(){    
  if(Serial.available() ) 
  {
    processSyncMessage();
  }
  if(timeStatus()!= timeNotSet)   
  {
    digitalWrite(13,timeStatus() == timeSet); // on if synced, off if needs refresh  
    digitalClockDisplay();  
  }
  delay(1000);
}

void digitalClockDisplay(){
  // digital clock display of the time
  Serial.print(hour());
  printDigits(minute());
  printDigits(second());
  Serial.print(" ");
  Serial.print(day());
  Serial.print(" ");
  Serial.print(month());
  Serial.print(" ");
  Serial.print(year()); 
  Serial.println(); 
}

void printDigits(int digits){
  // utility function for digital clock display: prints preceding colon and leading 0
  Serial.print(":");
  if(digits < 10)
    Serial.print('0');
  Serial.print(digits);
}

void processSyncMessage() {
  // if time sync available from serial port, update time and return true
  while(Serial.available() >=  TIME_MSG_LEN ){  // time message consists of a header and ten ascii digits
    char c = Serial.read() ; 
    Serial.print(c);  
    if( c == TIME_HEADER ) {       
      time_t pctime = 0;
      for(int i=0; i < TIME_MSG_LEN -1; i++){   
        c = Serial.read();          
        if( c >= '0' && c <= '9'){   
          pctime = (10 * pctime) + (c - '0') ; // convert digits to a number    
        }
      }   
      setTime(pctime);   // Sync Arduino clock to the time received on the serial port
    }  
  }
}

time_t requestSync()
{
  Serial.print(TIME_REQUEST,BYTE);  
  return 0; // the time will be sent later in response to serial mesg
}

Answer 1

Conrad and I discovered the solution after a 20 minute chat:

To set the variable to EST
TZ_adjust=-5;

sudo echo "T$(($(date +%s)+60*60*$TZ_adjust))" >/dev/tty.usbmodemfa131