The situation
Advice, a fitting, a workshop slot, a pickup: the customer should propose a time. If the proposal comes as free text, it says "next week in the morning".
An appointment request with fixed time slots
A preferred appointment is only usable once it fits the opening hours. That can be set on the field instead of sorted out afterwards.
The job
Where the built-in tools stop
A text field accepts any spelling, and none of them can be sorted. A date field without limits also accepts last Sunday.
This is what the form looks like
The time jumps in quarter hours, because the field carries a step size of 900 seconds.
What matters here
your-shop.example/admin
FormsCancelSave
FormStructureElement
Elements
#NameTypeWidth
Create top-level element
The grid sits in the field
Minimum, maximum and step become real attributes on the time field. The browser accepts 08:15 and refuses 08:07, before anything is sent.
No appointment in the past
A minimum date on the date field rules out yesterday. The server checks the same limit again afterwards.
Two columns instead of a line of prose
Date and time appear in the export as columns of their own. That lets you sort the list by day without reading it.
The form asks for a wish, it reserves nothing. It knows neither calendars nor capacity. Whoever needs a real booking with an availability check needs a different tool.
Where it sits
As a block on an appointments page, or through an assignment on the services that have appointments. With a phone number field instead of the branch it becomes a callback request form.
What comes back
Topic, branch, date and time as four columns. Optionally an email also goes to the address stored on the form.