After creating an annotation in PDFNet, if I then open it in another reader, it will change the date of the annotation. How can I prevent this from happening?
It is up to a particular PDF reader whether it decides to modify an annotation or not. There is no real way to control this from the PDF creation itself.
For example, adding a StickyNote/Text annotation using Adobe Acrobat, and then opening it in Foxit, Foxit will modify (including the date) the Acrobat generated annotation.