Tell your customer those features will add $50k and 3 weeks to the project cost, see how much they want them.
Or tell your customer how much more it will cost to write and maintain the "thousands" of lines of JS, and let them and let them say "nope".
They won’t say that, tho.
Everything is sales and everyone sells. If you can't sell your clients on why they shouldn't spend resources making a custom date picker, get a better salesperson in there.