Example

/*
 *  Copyright (c) 2012 Neal Horman - http://www.wanlink.com
 *  
 *  License: MIT open source (http://opensource.org/licenses/MIT)
 *      Summary;
 *      Use / modify / distribute / publish it how you want and 
 *      if you use it, or don't, you can't hold me liable for how
 *      it does or doesn't work.
 *      If it doesn't work how you want, don't use it, or change
 *      it so that it does work.
 */

#include "mbed.h"
#include "DS1307.h"
 
class I2C2 : public I2C
{
public:
    I2C2 ( PinName sda, PinName scl, int hz, const char *name = NULL) : I2C ( sda, scl, name) { frequency(hz * 100); };
};
 
I2C2 gI2c ( p28, p27, 100 );
RtcDs1307 gRtc ( gI2c );
Serial gSerial(USBTX, USBRX);
 
void printDT(char *pre, DateTime &dt)
{
    gSerial.printf("%s - %u/%u/%02u %2u:%02u:%02u\r\n"
                ,pre
                ,dt.month(),dt.day(),dt.year()
                ,dt.hour(),dt.minute(),dt.second()
                );
}
 
bool rtcUpdate(RtcDs1307 &rtc, int32_t bias) // this must be signed
{   bool bUpdated = false;
 
    // Use the compiled date/time as a basis for setting the clock.
    // We assign it to a signed integer so that negative biases work correctly
    int64_t compiledTime = DateTime(__DATE__,__TIME__).unixtime();
 
    // This assumes that the program is run VERY soon after the initial compile.
    time_t localt = DateTime(compiledTime + bias).unixtime(); // offset by bias
    
    // If the stored static time stamp does not equal the compiled time stamp,
    // then we need to update the RTC clock and the stored time stamp
    if(*((time_t *)&rtc[0]) != localt)
    {
        // Update the RTC time as local time, not GMT/UTC
        rtc.adjust(localt);
        // Store the new compiled time statically in the object ram image
        *((time_t *)&rtc[0]) = localt;
        // Push the object ram image to the RTC ram image
        bUpdated = rtc.commit();
    }
    
    return bUpdated;
}
 
int main()
{   time_t tick = 0;
 
    if(rtcUpdate(gRtc, -(5*60*60) )) // CDT
        gSerial.printf("Updated RTC to compiled time\r\n");
    gSerial.printf("compiled %s %s\r\n",__DATE__,__TIME__);
    DateTime timeFlashed(*((time_t *)&gRtc[0]));
    printDT("last flashed on",timeFlashed);
    DateTime dt = gRtc.now();
    printDT("reset",dt);
 
    gSerial.printf("rtc clock is %s\r\n", (gRtc.isRunning() ? "running" : "halted"));
 
    while(1)
    {
        if(time(NULL) >= tick)
        {
            dt = gRtc.now();
            gSerial.printf("%u/%u/%02u %2u:%02u:%02u          \r"
                ,dt.month(),dt.day(),dt.year()
                ,dt.hour(),dt.minute(),dt.second()
                );
            tick = time(NULL)+1;
        }
    }
}