Получение выражения не постоянно

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Получение нескольких ошибок из строки 56 в строку 67, говорящих "case expression not constant" и выражение должно иметь постоянное значение, но постоянное значение задается просто интересно, что пошло не так?

Что я уже пробовал:

#include <stdio.h>
#include <stdlib.h>

const int JAN = 1;
const int FEB = 2;
const int MAR = 3;
const int APR = 4;
const int MAY = 5;
const int JUN = 6;
const int JUL = 7;
const int AUG = 8;
const int SEP = 9;
const int OCT = 10;
const int NOV = 11;
const int DEC = 12;

const int DAYS_PER_MONTH = 30;
const int DAYS_PER_YEAR = 365;
const int MONTHS_PER_YEAR = 12;

typedef struct {
	int year;
	int month;
	int day;
} DATE;

void showDate(DATE* whichDate);
int daysFromTo(DATE* dateBegin, DATE* dateEnd);
DATE* getDateFromUser(char* dateDescription);
void addDaysToDate(int daysToAdd, DATE* dateToModify);

void testDates(void);

DATE* getDateFromUser(char* dateDescription)
{
	DATE* newDate = NULL;
	newDate = malloc(sizeof(DATE));
	printf("Please enter %s in MM/DD/YYYY format:", dateDescription);
	if (scanf("%d/%d/%d", &(newDate->month), &(newDate->day), &(newDate->year)) != 3)
	{
		printf("That wasn't in the requested format. Your date will be 01/22/1984\n");
		newDate->month = 1;
		newDate->year = 1984;
		newDate->day = 22;
	}
	return newDate;
	{

	}
}
void showDate(DATE* whichDate)
{
	char* monthName;
	switch (whichDate->month)
	{
	case JAN: monthName = "January"; break;
	case FEB: monthName = "February"; break;
	case MAR: monthName = "March"; break;
	case APR: monthName = "April"; break;
	case MAY: monthName = "May"; break;
	case JUN: monthName = "June"; break;
	case JUL: monthName = "July"; break;
	case AUG: monthName = "August"; break;
	case SEP: monthName = "September"; break;
	case OCT: monthName = "October"; break;
	case NOV: monthName = "November"; break;
	case DEC: monthName = "December"; break;
	default: monthName = "!?!?"; break;

	}
	printf("%s %2d, %4d", monthName, whichDate->day, whichDate->year);
}

int daysFromTo(DATE* dateBegin, DATE* dateEnd)
{
	int diffYears;
	int diffMonths;
	int diffDays;
	diffYears = dateEnd->year - dateBegin->year;
	diffMonths = dateEnd->month - dateBegin->month;
	diffDays = dateEnd->day - dateBegin->day;
	return DAYS_PER_YEAR * diffYears + DAYS_PER_MONTH * diffMonths + diffDays;
}

void addDaysToDate(int daysToAdd, DATE* dateToModify)
{
	printf("Adding %d days to ", daysToAdd);
	showDate(dateToModify);

	dateToModify->day += daysToAdd;
	if (dateToModify->day > DAYS_PER_MONTH)
	{
		dateToModify->month += dateToModify->day / DAYS_PER_MONTH; //Increase months by excess month(s) in days
		dateToModify->day %= DAYS_PER_MONTH; //Keep remaining days that don't exceed a month
	}

	if (dateToModify->month > MONTHS_PER_YEAR)
	{
		dateToModify->year += dateToModify->month / MONTHS_PER_YEAR; //Increase years by excess year(s) in months
		dateToModify->month %= MONTHS_PER_YEAR; //Keep remaining months that don't exceed a year
	}

	//Note: There'a problem with this logic. We'll get months numbered 0 to 11 instead of 1 to 12,
	// and days 0 to 29 instead of 1 to 30!

	printf(" results in ");
	showDate(dateToModify);
	printf(".\n");
}

void testDates(void)
{
	DATE* birthday = NULL;
	DATE* today = NULL;
	int distance;

	today = getDateFromUser("today's date");
	printf("You entered:");
	showDate(today); printf("\n");
	birthday = getDateFromUser("your birthday");
	distance = daysFromTo(birthday, today);
	printf("That's about %d days ago\n", distance);
	addDaysToDate(distance, today);
	printf("Just think: in %d more days, it'll be ", distance);
	showDate(today); printf("!\n");
}

int main() {
	testDates();
	return 0;
}

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