Thank you for your message.
I have just now tried using Document Attachment webparts and in transformation used:
<%# Eval("AttachmentLastModified") %>
And it did show the date as intented. Can you try to get the AttachmentID and then find this attachment in CMS_Attachment table in your database and see what values are stored for this attachment? What version of Kentico are you using? However this kind of functionality should not be touched by any of the hotfix as this is quite basic.
Are you able to provide any more relevant information to this issue?