A bit late for OP, but for people who land on this thread via search: my solution for this is a hack, but it works. I use the date range control since it's the only available date picker, and then add a metric called "Pick single date warning" defined as
IF(COUNT_DISTINCT(date) > 1, "SELECT A SINGLE DATE", "")
Then I add a "Scorecard" chart using this metric with the field name hidden and place it directly under the date picker. If a user selects a multi-date range they see the message, and it goes away when they have a single date.
I have used this method extensively when it's hard to create the perfect dataset and some user selections may yield invalid results.