Sample code from Setting Dates in JavaScript at the JavaScript/Java site at BellaOnline.com

setDate()

sets the date (day of month) in an existing date object
EXAMPLE: now.setDate(12)     

setMonth()

sets the month in an existing date object
EXAMPLE: now.setMonth(2)     

setFullYear()

sets the year (4 digit form) in an existing date object
There is also an obsolete setYear() function, but it should be avoided whenever possible
EXAMPLE: now.setFullYear(2008)     

setHours()

sets the hours portion of the time of day in an existing date object (uses a 24 hour clock)
EXAMPLE: now.setHours(14)     

setMinutes()

sets the minutes portion of the time of day in an existing date object
EXAMPLE: now.setMinutes(17)     

setSeconds()

sets the seconds portion of the time of day in an existing date object
EXAMPLE: now.setSeconds(24)     

setMilliseconds()

sets the milliseconds portion of the time of day in an existing date object
EXAMPLE: now.setMilliseconds(300)     

setTime()

sets the date and time of an existing Date object using seconds since (before for negative numbers) January 1, 1970
EXAMPLE: now.setTime(-999999999)