When using a date column in forms, it would be great to have the ability to restrict it to a specified set of dates. And it would also be great to have the added option to automatically update the specified set of dates by, for example, removing dates once they are in the past. I have several different workarounds for the former that I have used, such as listing the dates as text values in a dropdown list or using a different form for each of the dates and hiding the field in the form with a default value. However, it takes more time putting those workarounds in place and adds further complexity to the underlying sheet.