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I liked the simplicity of being able to adjust a date using @Adjust in Notes and wanted the same simplicity in Javascript. So here it is:

Date.prototype.adjust = function(yr,mn,dy,hr,mi,se) {
var m,t;
this.setYear(this.getFullYear() + yr);
m = this.getMonth() + mn;
if(m != 0) this.setYear(this.getFullYear() + Math.floor(m/12));
if(m < 0) {
this.setMonth(12 + (m%12));
} else if(m > 0) {
this.setMonth(m%12);
}
t = this.getTime();
t += (dy * 86400000);
t += (hr * 3600000);
t += (mi * 60000);
t += (se * 1000);
this.setTime(t);
}

var d = new Date();
d.adjust(years, months, days, hours, minutes, seconds);

Each argument is required, you pass a positive or negative number to adjust the date by that value or pass 0 for no change.

You can supply positive or negative numbers just like in @Adjust. It copes with year roll over if you, for example, add 15 months or -15 months to the date or even if it's December and you increment the month by 2.