off-by-one day errors + timezone errors when using SDC components with a date or date-time prop

Overview

Identified at #3550882-8: Empty (optional) date prop throws error upon reload, due to bug in client-side transform →

Given a SDC component with a date field.
After adding the component to a page, if I pick 12/10/2025, publish, and refresh the page, it renders as 12/09/2025. This might be an issue in the transform itself or some funny timezone/daylight timesaving issue:

Steps to reproduce

1. Enable sdc_test_all_props
2. Add all-props to a page.
3. On String, format=date-time, I set 27/08/2016 22:00
4. On String, format=date, I set 27/08/2016 (it's around 20:00 now at GMT+2)
5. Preview shows 2016-08-27T22:00:00.000Z and 2016-08-27 as ¿expected?
6. Forms shows 27/08/2016 22:00 and 27/08/2016 as expected.
7. Save and visit page. It shows the value expected as above.
8. Go to edit and select the component.
9. Preview shows 2016-08-27T22:00:00.000Z and 2016-08-27 as ¿expected?
10. Forms shows 28/08/2016 00:00 🤯 and 26/08/2016 🤯.

• So the first one lost the timezone (my /admin/config/regional/settings timezone says Madrid, my user profile has Madrid, my browser too, and with other values I verified is a gap of 2 hours, aka GMT+2 vs UTC),
• but the second is a plain off-by-one error, probably given to the JS - PHP conversion:
  • javascript dates starts at 0: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global...
  • php dates starts at 1: https://www.php.net/manual/en/datetime.format.php

Proposed resolution

There are 2 hard problems in computer science: cache invalidation, naming things, and off-by-1 errors, and I'm never late to meetings, you created the appointment in the wrong timezone.

User interface changes

TBD