File content as of revision 0:897d43c0dcf0:

#include "mbed.h"

Serial pc(USBTX, USBRX);
DigitalOut myled(LED1);

void resetClockTickCounter () {
    LPC_RTC->CCR |= (1<<1); // CTC Reset
    LPC_RTC->CCR &= ~(1<<1);
}

void calibConfig (int value, int dir) {
    if (dir) {
        LPC_RTC->CALIBRATION = (value & 0x1ffff) | ((dir == -1) ? (1<<17) : 0);
        LPC_RTC->CCR &= ~(1<<4); // Calibration counter disable.
    } else {
        LPC_RTC->CALIBRATION = 0;
        LPC_RTC->CCR |= (1<<4); // Calibration counter disable.
    }
}
extern "C"
void RTC_IRQHandler () {
    time_t sec = time(NULL);
    struct tm *t = localtime(&sec);

    myled = !myled;
    pc.printf("%d  %04d-%02d-%02d %02d:%02d:%02d\r\n", sec,
        t->tm_year + 1900, t->tm_mon + 1, t->tm_mday, t->tm_hour, t->tm_min, t->tm_sec);
    LPC_RTC->ILR |= (1<<0);
}

int main() {

    pc.baud(115200);
    pc.printf("RTC Calibration\r\n");
    set_time(1514732400); // 2018-01-01 00:00:00
    LPC_RTC->CIIR |= (1<<0);
    NVIC_EnableIRQ(RTC_IRQn);

    for (;;) {
        if (pc.readable()) {
            char c = pc.getc();
            switch (c) {
            case '0':
                calibConfig(0, 0);
                pc.printf("0\r\n");
                break;
            case 'f': // Forward
                calibConfig(4, 1);
                pc.printf("F\r\n");
                break;
            case 'b': // Backward
                calibConfig(4, -1);
                pc.printf("B\r\n");
                break;
            case 'r': // Reset
                resetClockTickCounter();
                pc.printf("R\r\n");
                break;
            }
        }
    }
}