Without a lot of programing, LiveCycle Designer provides this as part of its forms. For AcroForms one needs to create a special series of fields and scripts to perform this function. You can download an AcroForms example from [url=http://www.pdfscripting.com/public/programs/downloadsearch.cfm?searchtype=simple&searchmode=cat&keywords=recent&sortby=name&cat=Date%20and%20Time]All Files in Date and Time Category[/url] and scroll down to the "*Popup Calendar for PDF Forms*". Note you will have to have a paid membership to access this resource.
George Kaiser