file Last modified date will change when parent folder name has change

Dear All,

we try to create a web page to get the file Last modified date, but don't know why when we modified the parent folder name then the file Last modified date will change.

Steps to reproduce the issue:

         1. Upload a file to sharepoint at 2013.02.01

         2. Rename the file's parent folder at 2013.02.02, the file's 'Last modified date' in sharepoint website is 2013.02.01, it's correct.

But when we use SPFile.TimeLastModified to get the date, it's 2013.02.02, it's wrong.

         3. Rename again the file's parent folder at 2013.02.02, the file's 'Last modified date' is changed to 2013.02.02, both sharepoint website & API are wrong.

         however, user expect the 'Last modified date' is still

2013.02.01 whenever they rename the parent folder name. is have anyway to fix it?