To use the app, you don't need to have WhatsApp installed or even have a specific contact saved in your phone. The app only needs you to enter a phone number and once you do, you can see information about every time that particular number has signed on and off WhatsApp.
From the moment you start using the app, a calendar is created that shows you the records, organized by hours and days, and statistics related to the time of connection of the phone number. That way, you can find out how many times the person has connected in the last 24 hours, week, or 30 days and even how many minutes they spent connected.
In addition, WhatsDog let you decide if you want to be notified the exact moment the person connects.
This app is no longer updated and therefore may not work with the newest versions of WhatsApp.