The ACF Javascript API does not allow the setting of a datepicker date programmatically.
acf.getField('the_field_id').val([the date]); // does not work
// nor does set()
Nor do any of the usual workarounds to change the value of the jQuery UI datepicker.
Surely they wouldn’t have written a datepicker object where you can’t programmatically change the date?
Does anyone have a workaround?
This might be a bug. I had a similar issue with image fields some time ago. I don’t have a work-a-round. You might want to contact the developer.