How can I save user input permanently with javascript?

Question

So I am making an countdown clock extension for chrome, and I want to save the user input so they don't have to enter their date everytime they open the extension, is there anyway to do this? Here is my code

// Set the date we're counting down to
var userInput = prompt ("Enter Your Date");
var countDownDate = new Date(userInput).getTime();

Answer 1

https://developer.mozilla.org/en-US/docs/Web/API/Storage

Answer 2

Same question today
https://www.sololearn.com/Discuss/1224621/how-do-you-make-a-variable-that-still-keeps-it-s-value-even-after-you-close-the-code