How to auto-refresh the page if the bx-rand in the header got a more recent last-modified?

Question

JavaScript

I made the pages cache in the browser until 8 pm, but I would like the page to load a new page when the content changes. The site is on Bitrix and there is a composite that sends a background subrequest with the argument ?bxrand={timestamp} in the header of which you can see the last-modified header and compare its value with the date header of the current page.
Now the question is, how can this be compared and how can I force the page to update if date is older than last-modified?
If the page is updated to f5, it will load the new data itself, I need it to be updated without f5.
I’ll also add an explanation, I don’t need a cycle, I need the page to load, the script placed on it makes a subrequest, and if this page is relevant, then everything remains as it is, if not, then the page is updated.
I apologize in advance, I understand that this can be done not with a large js script, but js is just starting to study, my profile is the server part. Therefore, it’s not difficult for anyone to kick as possible in a more accurate direction, mutual assistance is possible, you tell me with js, I’ll tell you with the server)