you may want to look into creating a customization based on a custom user profile. The user profile could store the date and your customization code would populate the custom user profile every time the user has signed in. There is a freemarker event context object that you can use for that. You can file a request to support for adding a custom user profile field.
If you are not sure on how to build such customization, you may also want to get in touch with your Lithium Manager and scope this up as a custom engagement.
Thanks,
