Rules: Send mail to author when value in date field has passed So, I have a content type "contest" which has an "end date" field. I want to setup rules so the author of a "contest" gets an e-mail as a reminder that the contest ended.

As event I used "Cron maintenance tasks are performed" and as condition I try to use "Data Comparison" to check the value of the end date to be smaller than current site time. The problem is I can't use the token of the "end date" field as my event is not node-related. I tried to add a "Content is of type" condition, but also here, I can't select anything node-related.

I feel like overlooking something small but I've been searching now several hours and could use some help..
Assuming you are using Drupal 7. 

You need to enable the Rules Scheduler module, it comes with Rules.

Then for Rule event use After saving new content. 

Add condition, content is of type: Contest

Exit out of your Rule, and now create a Rule Component of Data Type: Node 

Add condition, content is of type: Contest

Action Send E-mail.

Now exit the component and go back to your rule, for action, schedule rule component, and use your end-date field for schedule evaluation date.

I highly suggest you watch Introducing Rules Scheduler and I am sure my answer will make more sense to you.