Learning C: with Dan Gookin's "Tiny C Projects"

Chapter 2: Daily greetings

this is the moon phase code I studied this evening from Gookin's Git:

#include <stdio.h>
#include <time.h>


int moon_phase(int year,int month,int day)
{
    int d,g,e;


  d = day;
    if(month == 2)
    d += 31;
    else if(month > 2)
    d += 59+(month-3)*30.6+0.5;
    g = (year-1900)%19;
    e = (11*g + 29) % 30;
    if(e == 25 || e == 24)
    ++e;
  return ((((e + d)*6+5)%177)/22 & 7);
}


int main(int argc, char *argv[])
{
  time_t now;
  struct tm *clock;
  char time_string[64];
  char *phase[8] = {
      "waxing crescent", "at first quarter",
      "waxing gibbous", "full", "waning gibbous",
      "at last quarter", "waning crescent", "new"
  };
  int mp;


  time(&now);
  clock = localtime(&now);


  strftime(time_string,64,"Today is %A, %B %d, %Y%nIt is %r%n",clock);


  printf("Greetings");
  if( argc>1 )
    printf(", %s",argv[1]);
  printf("!\n%s",time_string);
  mp = moon_phase(clock->tm_year+1900,clock->tm_mon,clock->tm_mday);
  printf("The moon is %s\n",phase[mp]);


  return(0);
}