May be this problem is only Mac related, due a hidden or forgotten CHECK BOX after the text Due date and before the Date.
To add Due Date on Mac:
- You can create a Due Date with Automator. Make a workflow with Create a reminder, run it and set a Due date different to the 'Alert date'.
- The 'Remind Me' app (free on Mac App Store) can do so too.
- Other non-Apple apps that create non Apple-Standard info in the reminders, in this case, a 'Due date' different from the 'Remind me on a day' date. For example, testing it with GoodTask if I set a 'Due date', GoodTask activate the Alert checkbox too. In Apple Reminders is shown as usual: 'Remind me on a date: ...' If I uncheck the 'Due date' in Goodtask, the 'Alert' field goes off too. But if I uncheck only the 'Alert' checkbox, letting the 'Due date' activated, the same reminder in Apple Reminders changed to 'Due date' without the 'Remind me on a day' checked.
You can add both Remind me on a date and Due Date on iOS Reminders- This is a Mac missing feature (Mac OS 11.12.6, I don’t know if this was fixed on High Sierra).
To delete the Due Date on Mac:
A missing checkbox could disable de Due date, but it’s not possible without that checkbox.
- Making match both date and time (as @user11633 said) you can get rid of the Due date.
- You may look if you created that reminder in other app (BusyCal, GoodTask, Fantastical...) If you do so, you can delete the 'Due date' on that app.
- Mac app Reminders have a bug because if you have Remind me on a date and Due date, check the Remind me on a location, select any option (When I Arrive, or When I Leave), then, click to uncheck Remind me on a day it deletes the Due Date, keeping the alert on. It's crazy.
By the way, I've discovered that you can change, in Mac, both the 'Due date' and the 'Remind me' date dragging the reminder to the calendar on the bottom left, or drop it into the 'Scheduled' field (at top of the Lists sidebar) to set the date to Today.
If you select the due date clicking on the numbers, or with Tab key, then you can change the date and time with cursor up/down. The changes can (some times not) affect to both Due Date and Remind Me (keeping the time difference).
In menu Edit it shows: Undo changes in 'Due Date', but there's no visible way to 'unckeck' the Due Date field. This is a proof that the feature exists, but a the checkbox is missing.