How to change last modified date in Attachment List

Hi All, I am using an Attachment List that is binded to a Temporary Attachments Provider to show the files I get when I call a webservice which gets some files that are in the Integration Server.
I am having a problem when I see the Last Modified Date it always shows the actual date. I know how to get it from the IS but I do not know how to set last modified date to Attachment Items.
Does anyone know how to do it?

Thanks in advanced,