Adding an automatic shipping time metafield

Hi,

I am wanting to add a metafield into my product page called: Estimated Shipping Times.

And I would like to know if (and how) to use the Json option to create a formula that will take today’s date, add 20 days, and then show the shopper that date.

I,e if the shopper sees the product on the 1st of June, the meta field would print out the 21st of June.

Thank you in advance for any help!