Can I write an email campaign now and have it sent automatically later?

Yes.

1. Go to Marketing → Email Campaigns and open or create the campaign.
2. Fill in the Campaign name, Subject line and Audience, then click Next: Design the email → and write the email.
3. The design step opens full screen, so click ✕ Exit full screen, then under Schedule for later pick the date and time.
4. Click Schedule.

The time must be in the future. The campaign shows as Scheduled in the list and starts sending within about a minute of that time. You can still edit it until then.