Add a number of days to datepicker value

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Hi All.
I’m trying to add a number of days to the date selected by a user in a datepicker. I’ve found some code on here managed to do this with current date:

let expireDate = new Date();
expireDate.setDate(expireDate.getDate() + 10);

But I cannot seem to work out how to do the same with a date selected from a datepicker.

Any ideas? Thanks is advance.