When you draw your values from a table, Filemaker insists on sorting the values in ascending order as you've found. There are no built in features that will allow you to specify a different order.
Comment has posted a description of a work around for relatively modest (up to a hundred?) values.
You might also display your values in a portal where clicking a portal row selects the date. THen you can sort your values however you need, but you lose screen real estate as you no longer have a drop down or pop up to use in selecting the value.