Default text until date entered

So first, fields with the same names have the same value, so just make the fields you want copied have the same name

For the other bit, unfortunately you have a conundrum. The correct way to display instructional text in a field is with the format event. This event provides an opportunity to change the display of the field without changing the value.  But since you've already selected the Date formatting, this option is not available.

There is no way around this if you want to keep the calendar popup.

But if you don't mind loosing it, There is a way around this, but it's tricky.  First, you have to set the Custom formatting to override the display on an empty field

// Custom Format Script

if(event.value == "") event.value = "This is my message";

Make sure this works first, Then to get the automatic data popup, you have to enter the built-in JavaScript date function into the custom Keystroke script, with the correct formatting options. Once you do this the Format Tab on Field Properties will revert to Date Formatting.

// Custom Keystroke Script

AFDate_KeystrokeEx("mm/dd/yy");