The Day and Week Planners do not let you schedule for the present day (today). You can only schedule the Planners for tomorrow or a later date.
The Planners are designed to schedule posts from 00:00 hrs to 23:59 hrs.
If the Planners let you schedule for today, some of the posts won’t be scheduled as their scheduled time would be in the past. This is why, Planners can be used only for the next day (or a later date).
If you do want to plan for today, you can do this manually by sharing products from your Products page.