"First seen", "Last seen" and "Last tried" date formatting is determined by your settings in the Lansweeper configuration console. Only the date format for Last Logon information is derived from the web server.
The date format has to be set for the user your IIS application is running under. To find out which user that is, have a look at the user name specified for your IIS executable in Windows Task Manager. Then go to your registry editor and find that same user under HKEY_USERS. You need to change the date formatting for this particular user. You can find the sShortDate value under HKEY_USERS/(Your User)/Control Panel/International.
For more information on users you might encounter in HKEY_USERS, please refer to
this knowledge base article.