How To: Convert Day Number to Month and Day

I needed to convert the day number, that is, what day it is between 1 and 365, into a month and day without using Calendar / DateTime classes and such today, so I knocked up a few simple functions that work assuming it’s not a leap year (i.e. that February has 28 days).

Here they are if they’re of any use to you:

int dateToDayNumber(int month, int day)
{
	// Catch invalid input and return early
	if (month < 1 || month > 12 || day < 1 || day > 31) return 0;
 
	if (month == 1 ) return       day;
	if (month == 2 ) return 31  + day;
	if (month == 3 ) return 59  + day;
	if (month == 4 ) return 90  + day;
	if (month == 5 ) return 120 + day;
	if (month == 6 ) return 151 + day;
	if (month == 7 ) return 181 + day;
	if (month == 8 ) return 212 + day;
	if (month == 9 ) return 243 + day;
	if (month == 10) return 273 + day;
	if (month == 11) return 304 + day;
	return 334 + day;
}
 
int dayNumberToMonth(int dayNumber)
{
	// Catch invalid input and return early
	if (dayNumber < 1 || dayNumber > 365) return 0;
 
	if (dayNumber <= 31 ) return 1;  // Jan
	if (dayNumber <= 59 ) return 2;  // Feb
	if (dayNumber <= 90 ) return 3;  // Mar
	if (dayNumber <= 120) return 4;  // Apr
	if (dayNumber <= 151) return 5;  // May
	if (dayNumber <= 181) return 6;  // Jun
	if (dayNumber <= 212) return 7;  // Jul
	if (dayNumber <= 243) return 8;  // Aug
	if (dayNumber <= 273) return 9;  // Sep
	if (dayNumber <= 304) return 10; // Oct
	if (dayNumber <= 334) return 11; // Nov
	return 12;                       // Dec
}
 
int dayNumberToDayOfMonth(int dayNumber)
{
	// Catch invalid input and return early
	if (dayNumber < 1 || dayNumber > 365) return 0;
 
	if (dayNumber <= 31 ) return dayNumber;       // Jan
	if (dayNumber <= 59 ) return dayNumber - 31;  // Feb
	if (dayNumber <= 90 ) return dayNumber - 59;  // Mar
	if (dayNumber <= 120) return dayNumber - 90;  // Apr
	if (dayNumber <= 151) return dayNumber - 120; // May
	if (dayNumber <= 181) return dayNumber - 151; // Jun
	if (dayNumber <= 212) return dayNumber - 181; // Jul
	if (dayNumber <= 243) return dayNumber - 212; // Aug
	if (dayNumber <= 273) return dayNumber - 243; // Sep
	if (dayNumber <= 304) return dayNumber - 273; // Oct
	if (dayNumber <= 334) return dayNumber - 304; // Nov
	return dayNumber - 334;                       // Dec
}

A touch cheap, but it gets the job done.