Instead of actual values, the "Last Login" and "Last Activity" fields in the user information dialog (User Manager -> Edit User -> Information tab) always report DateTime.MinValue for all Active Directory users.
By default, accessing the ActiveDirectoryMembershipUser.LastLoginDate and ActiveDirectoryMembershipUser.LastLoginDate properties throws NotSupportedException.
In order to avoid the exception, Sitecore Active Directory module returns DateTime.MinValue for both properties.
Please refer to the following MSDN articles for details:
https://msdn.microsoft.com/en-us/library/system.web.security.activedirectorymembershipuser.lastlogindate(v=vs.110).aspx
https://msdn.microsoft.com/en-us/library/system.web.security.activedirectorymembershipuser.lastactivitydate(v=vs.110).aspx