David,
I could see the answer is not so simple if you need to ensure valid dates, if you want to generate short date ranges like in your example it can be trivial
<$list filter="[range[.03,.09,.01]removeprefix[0.]addprefix[2020-03-]]">
</$list>
See how I use decimal numbers and remove prefix to have the result have a leading zero (below 10)
However personally I would keep dates as simple numbers
<$list filter="[range[.01,.31,.01]removeprefix[0.]addprefix[202003]]">
</$list>
because then you can ask if one is greater than the other and reformat it later
True YYYY-MM-DD can also be compared < >
The view widget can translate a value in a field into and date format you want to display it, leave the display format to the end where possible, as this maintains your choice and keeps it simple.
I still feel we need the ability to feed a date <variable> into the view widget.