The contact was saved in your Google Contacts? I'm not a WhatsApp user so I have very little knowledge about it, but I'd have to assume that WhatsApp must have permission to read and write contacts, and that WhatsApp has a trigger to automatically add contacts. If it bothers you, you could revoke the permission from WhatsApp or find the setting to turn off automatic contacts.
As far as finding the time of the contact, you'd have to contact the developer of WhatsApp and see if there's a log for app activity being written.