What are the forensic limitations of Metadata & Exif Data+ Adobe Photoshop 3 question; e.g. Metadata

I have some questions regarding metadata, and exif data on digital  cameras and its forensic limitations in Adobe photoshop 3;

For instance; After a picture is taken and has a metadata/exif digital  stamp of the "picture being taken day" under what circumstances can that  "picture being taken day" be changed?

Does the picture being taken day change when you import it into adobe  photoshop 3 and add a caption?

Does it change only when the picture is edited, to perhaps add a white  sidebar that says "family photo" or whatever?

Under what circumstances does the exif/metadata digital time stamp of  the "picture being taken day" change?

Also, is it possible to change the picture being taken day on a  photograph in adobe photoshop 3 manually?

Thanks-
Phil