seems like this is a known issue and currently there is no hotfix available for this, under some scenario's you face this kind of issues:
For your information, the file modified time in Explorer view is not correct if the following conditions are true:
- The client is using DAV (i.e, if the Web Client service is started in the client machine). And the web application is running on port 80 to use DAV (The port 80 requirement is only applicable for XP client).
- The file was uploaded in Explorer view initially. (By drag and drop / copy and paste).
There are two workarounds now and either one is breaking one condition of the issue:
- Use the 'All Documents' view to view the modified times
- When uploading the file, upload the file in 'All Documents' view. Article for reference: https://social.technet.microsoft.com/Forums/en-US/22c73b99-cab9-423b-adef-f4d861d514cc/modified-date-of-files-incorrect-when-accessing-sharepoint-library-as-a-mapped-drive?forum=sharepointadminlegacy
Thanks & Regards,