The field that drop-down calendar is attached to determines the functionality. If you attach the calendar to a date field, it will only insert the date selected. If you attach the calendar to a timestamp, it will insert the date selected and the current time.
You have it attached to a timestamp field. Change the field to date (but be sure the data is corrected first). :^)
BTW, if from menu in your file, you select Help, you can search and find answers. In this instance, the answer is documented if you search for drop-down calendar.
Thanks LaRetta, I eventually figured it out.