DIY Aeroponics Arduino Timing

unsigned char relayPin[12] = {4,5,6,7};//Remainining pins: (22,23,24,25,26,27,28,29}; // Define the led's pin


//initializes the function: Zone
void Zone(unsigned long on, unsigned long off, int i);




int mode[4]={LOW,LOW,LOW,LOW}; //Initilizes mode array  to LOW (OFF) Mode
unsigned long previousMillis[5]={0,0,0,0,0}; //initilizes previousMillis array
//Conversions
unsigned long Sec = 1000UL;
unsigned long Min = 60000UL;
//unsigned long Hour=60*Min;


//unsigned long daytime=43200000UL;


void setup()
{
  //Initilizes the Pins as OUTPUT going through the for loop. 
  int i;
  for(i = 0; i < 4; i++)
  {
    pinMode(relayPin[i],OUTPUT);
    digitalWrite(relayPin[i],mode[i]);
  }
}


void loop(){
  Zone(3*Min,15*Min,0);
  Zone(6000UL,900000UL,1);
  Zone(4000UL,900000UL,2);
}


/*
Zone:
Recieves: On time, Off time, and Pin Number(0-4)
Note: millis() function calls time since arduino has been on, in milliseconds (ms)
psuedocode: 
if (millis) - previlousMillis[i] is greater than (Off Time) and mode is equal to LOW
  then set previousMillis[i] to (millis), set pin mode to HIGH(ON), 
then it does it again for the ON time
*/
void Zone(unsigned long on, unsigned long off, int i){
  if (millis() - previousMillis[i]>= off && mode[i]==LOW){
    previousMillis[i]=millis();
    mode[i]=HIGH;
      digitalWrite(relayPin[i],mode[i]);
  }
  if (millis()-previousMillis[i]>= on && mode[i] == HIGH){
    previousMillis[i]=millis();
    mode[i]=LOW;
      digitalWrite(relayPin[i],mode[i]);
  }
}