Is there any way to stop the autofilling of todays date when the field is initially blank?
No, this is how the datepicker object works but you can get around this by prepopulating the default value with today's date and time. If you don't put in a default date, as soon as you click the arrow to put in today's date you trigger the action for date changed since you went from no date to today's date. If you put in a default date of today and click the arrow then the date change action will only trigger if you change the date from today. This will work much better and cleaner than my workaround above.