Episode scheduling is available as part of the Professional and Plus ZenCast plans.
On the episode creation page, scroll down to the "Publication date and time" section and click the calendar icon:
You can then select the date and time you'd like your episode to be published.
If no date is selected, or if a date in the past is selected, then the episode will be published immediately.
Do note that although ZenCast will publish your episode at the date and time specified, there can be a small delay until each podcast platform checks your Podcast feed and detects your newly published episode.