Add a function to add date start/end in the existing URL

Need to build a icon in a existing web page. When user press it, it pop up a dialog box to ask either the "start date & end date" or "how many last days". After web users enter "start date & end date" or "how many last days" and process "OK", the existing url will be reloaded with the generated string like "&startdate=20090815&enddate=20100118" at the end. The default "end date" is yesterday, "start date" is 14 days from today, "how many last days" is 14 days.