Countdown Timer and makeTime()

I’m have a RTC and I want to make a countdown timer. The time remaining will be displayed on an LCD matrix in hh:mm:ss format. I’m having trouble figuring out the number of seconds from now to the countdown time. I apparently lacking in some understanding of how makeTime(), and epochtime() work.

My RTC is set with the right time, I ran one of the example sketches that outputs the time and it was right. My intent is to set my countdown time using makeTime(), then subtract the now.unixtime(). I thought this would give me the number of seconds until I reached the countdown, but I’m getting numbers that are way too big.

Initially I was setting tm.Year = 2013, then I read it should be 2013 - 1900, but that didn’t fix it. I’m also not sure if tm.Month is 11 for December or 12. Neither way got things working.

Below is my test sketch

#include <Wire.h>
#include "RTClib.h" 
#include "Time.h"   

RTC_DS1307 RTC;

uint32_t refreshTimer; 
tmElements_t tm;

void setup() {
  Serial.begin(9600);
  Wire.begin();
  RTC.begin();
  refreshTimer = millis();
}

void loop()  {
  time_t nextMooverTime;
  uint32_t countdownSec;
  
  if ((long) (millis() - refreshTimer) > 0 )
  {
    DateTime now = RTC.now();
    
    tm.Second = 0; 
    tm.Hour = 10;
    tm.Minute = 30;
    tm.Day = 7;
    tm.Month = 12;
    tm.Year = 2013 - 1900;
    nextMooverTime =  makeTime(tm); 
    countdownSec = nextMooverTime - now.unixtime();
    Serial.print(nextMooverTime);   
    Serial.print("\t");
    Serial.print(countdownSec); 
    Serial.print("\t");
    Serial.print(now.unixtime());  
    Serial.println();
    
   // at 8:55 AM EST on 12/6/13, I get this
   // 3595401000	2209080988	1386320012
      
   // if I change month to 11, I get this
   // 3592809000	2206488929	1386320071
   
   // change year to 2013 instead of 2013 - 1900
   // 2708568104	1322247995	1386320109

    refreshTimer = millis() + 1000; // update display every 1/2 second
  }

}

I think I found the problem, the year should be tm.Year = 2013 - 1970, I was using 2013 - 1900. I'll test it later and confirm.