Prototype ¬ Detail

author: orangechicken [+], Submitted: 10.14.02 6a

// Requires daysInMonth array and leapYear function to correctly calculate the dates
Date.prototype.daysPerMonth = [31,28,31,30,31,30,31,31,30,31,30,31];
Date.prototype.leapYear = function ( year ) {
	if (year == null) { year = this.getFullYear() }
	return (((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0)) ? 1 : 0;
}
Date.prototype.nextDate = function(){
	var d = this.getUTCDate();
	var m = this.getUTCMonth();
	var y = this.getUTCFullYear();
	
	var maxDays = this.daysPerMonth[m];
	if( m == 1 ) maxDays += this.leapYear(); // february
	
	d++;
	
	if( d > maxDays ) {
		d = 1;
		m++;
		if( m > 11 ){
			m = 0;
			y++;
		}
	}
		
	return new Date( y, m, d );
}
Date.prototype.previousDate = function(){
	var d = this.getUTCDate();
	var m = this.getUTCMonth();
	var y = this.getUTCFullYear();
	
	var maxDays = this.daysPerMonth[m > 0 ? m - 1 : 11 ];
	if( m - 1 == 1 ) maxDays += this.leapYear(); // february
	
	d--;
	
	if( d <= 0 ) {
		d = maxDays;
		m--;
		if( m < 0 ){
			m = 11;
			y--;
		}
	}
	
	return new Date( y, m, d );
}

usage

today = new Date();
yesterday = today.previousDate();
tomorrow = today.nextDate();

trace( today );
trace( yesterday );
trace( tomorrow );

/* outputs:
Mon Oct 14 03:31:16 GMT-0700 2002
Sun Oct 13 00:00:00 GMT-0700 2002
Tue Oct 15 00:00:00 GMT-0700 2002
*/