Angular Ui datePicker is in invalid state

I have a form developed with angular 5,angular cli,html-css and bootstrap.In my form i have a datepicker.When I submit the form all the details of the form should be submitted to "db.json" file of rest api.Now when  I am trying to edit the form by making use of put request "Date picker field is going to invalid state".Someone help me how to solve this issue.

Note:according to my references angular stores date in  form of object and json stores date in the form of a string.How to get that string formatted date in the form of object at the time of editing(from ISO8601 to dd/mm/yyyy )