HTTP Certificate Checker
Data security is very important nowadays. This is why all your websites and services should by encrypted using https. Though there is one thing that is more unprofessional than not having the traffic between your customer and you secured: this is an invalid secure policy or certificate. When having an expired https certificate the browser goes crazy. It tells the user that he should not visit this page because there is something wrong. So apparently this is much worse than having no encryption at all.
One of the main reasons why an certificate is not valid is the expiration date. The most certificates are valid between 90 days and one year. After that period you have to authenticate yourself against a certification authority. When forgetting to renew the certificate it is automatically going to become invalid and the browser will know immediately. That is why you should use leankoala to check your certificates for their expiration date and lets it report an upcoming renewal to your favorite channel like slack.