CCS/MSP430F6721: Problem calculating seconds since epoch from calendar date and time

Tool/software: Code Composer Studio

Hi,

I am trying to implement a simple function to calculate seconds since epoch(1970-01-01 00:00:00) since MSP430F67xx MCU don't have counter mode in RTC modules. The following is the formula I used:

tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 + (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 - ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400

As exampled in code attached below, with hard coded year/month/day/hour/minute/second, the calculated seconds as output from MSP430 seems always smaller than the number I manually calculated in calculator by 65536 or 0x00010000.

If hard coded calendar as 2016-06-25 20:59:12, the seconds calculated by MSP430 is 1466736416/0x576C9F20 while the correct value should be 1466801952/0x576D9F20

What I am missing here? Any comment or feedback will be appreciated.

    //WDTCTL = WDTPW | WDTHOLD;										// Stop watchdog timer	sysInit();
    WDTCTL = WDTPW | WDTSSEL__ACLK | WDTIS_3;						// Enable watchdog timer: ACLK, 4min and 16 seconds
    initSystem();
    unsigned long seconds = 0;
    char output[5], i;
    int year, month, day, hour, minute, second, dayOfYear;

    while (1) {
    	 year = 116;
    	 month = 6;
    	 day = 25;
    	 hour = 20;
    	 minute = 59;
    	 second = 12;
    	 //dayOfYear = getDayOfYear(month, day, RTCYEAR);
         dayOfYear = 175;
    	 seconds = second + minute * 60 + hour * 3600 + dayOfYear * 86400 + (year - 70) * 31536000 + ((unsigned int)((year - 69) / 4)) * 86400 - ((unsigned int)((year - 1) / 100)) * 86400 + ((year + 299 ) / 400) * 86400;
    	 //seconds = calenda2Seconds(month, day, RTCYEAR);
    	 output[0] = RTCYEAR >> 8;
    	 output[1] = RTCYEAR;
    	 output[2] = month;
    	 output[3] = day;
    	 output[4] = hour;
    	 output[5] = minute;
    	 output[6] = second;
    	 output[7] = dayOfYear >> 8;
    	 output[8] = dayOfYear;
    	 output[9] = seconds >> 24;
    	 output[10] = seconds >> 16;
    	 output[11] = seconds >> 8;
    	 output[12] = seconds;
    	 for(i = 0; i < 13; i++) {
			while (!(UCA1IFG&UCTXIFG)); // USCI_A0 TX buffer ready?
			UCA1TXBUF = output[i];
    	 }
    	 __no_operation();
    }