Store The Values Entered During Runtime In The Select Box?

I have a web page that has a select box and a textbox to add data into the select box. What I want to do is when I enter some data say a "valid date" in the textbox and click on Add button it should copy the same to the textbox such that ,when I navigate to a different link in the web interface and visit the same page again, the value that I had entered earlier should remain in the select box and be visible.

I have been able to copy the data from the textbox to the select box, but when I revisit the same page the value entered earlier does not remain in the page. Once the values are stored in the select box, those values in the select box should be copied to a structure written in C language in the backend.Can anyone tell me how can I store the values entered during runtime in the select box using javascript.