Adding to the thread because I ran into exactly this when setting up a weekly “design feedback” topic for a client community.
What worked for us: we ended up using a webhook trigger from an external n8n workflow rather than the built-in automation scheduler. The n8n side handles the date interpolation (you have full control over date formatting there), and then it calls the Discourse API to create the topic with the pre-formatted title. A bit more infrastructure, but it means you are not fighting against the automation placeholder system.
That said, if you want to stay inside Discourse automation entirely: the placeholder system @NateDhaliwal flagged in the source is the right place to look. The placeholder declarations in the topic script are what get exposed to the title and post fields. The challenge is that {{date}} is not a built-in placeholder — those are user-context ones like creator_username. Adding a date placeholder would require either a plugin or a PR to discourse-automation.
For @Eingang wanting formatted dates specifically: the external workflow route gives you full moment.js-style control. We format ours as MMMM YYYY before sending to the API and it comes through exactly as written in the topic title. Not the most elegant solution but it is reliable and has been running without issues for about four months now.