Thank you for your patience.
We have analyzed your query “Any way to display alert dialog before change the value in dateitem” and as of now DataForm doesn’t have direct support for this, but you can achieve this by creating custom editor and using CustomRenderer for the editor in your sample.
Kindly refer our KB for more information about the same, by referring the following link:
In this KB we have shown popup while opening entry, and we suspect that your requirement is same scenario with DatePicker, kindly confirm the same.
Kindly revert us if you have any concern.
Arun Siva R