File content as of revision 0:9f741d395d17:

#include "mbed.h"

Serial serial(USBTX,USBRX);

void serialISR();  // ISR that is called when serial data is received
void setTime();    // function to set the UNIX time
int setTimeFlag = 0;  // flag for ISR

char rxString[16];  // buffer to store received string

int main()
{
    serial.attach(&serialISR);  // attach serial ISR
    char buffer[30];  // buffer used to store time string
    
    set_time(0);  // initialise time to 1st January 1970

    while(1) {

        time_t seconds = time(NULL);  // get current time
        // format time into a string (time and date)
        strftime(buffer, 30 , "%X %D", localtime(&seconds));
        // print over serial
        serial.printf("Time = %s\n",buffer);
        wait(1.0);  // delay for a second
        
        if (setTimeFlag) {  // if updated time has been sent
            setTimeFlag = 0;  // clear flag
            setTime(); // update time
        }
    }

}

void setTime() {
    // print time for debugging
    serial.printf("set_time - %s",rxString);
    // atoi() converts a string to an integer
    int time = atoi(rxString);
    // update the time
    set_time(time);
}

void serialISR() {
    // when a serial interrupt occurs, read rx string into buffer
    serial.gets(rxString,16);
    // set flag
    setTimeFlag = 1;    
}