TrackSSL is a web service that regularly checks your SSL certificate for common errors
Add your certificates through the web interface and you'll receive email notifications when there are problems such as a pending expiry, or a misconfigured host.
Not just expiry
Ensure that infrastructure changes aren't changing your certificates under your nose. Get a notification when certificate information is updated.
Control your notifications
You decide what types of notifications you want and when. Integrate with Slack and get your notifications straight into your #devops channel.
Using Cloudflare? You probably don't want the change notifications enabled.
Using Let's Encrypt? You probably don't want to be notified that your certificate expires in 60 days.