How to change the permalink

OK, so here’s my issue:
On my single.php page, I want to change the permalinks of my post from:

https://mysite.com/my-post-title

to

https://mysite.com/#4

The posts have a date custom field (start_date) added to them. This is aside from the post date; e.g., I have a post with a start_date of 3-5-1964 that was actually posted after a post with start_date of 3-5-1985, but I want to change the permalink of the post with start_date 3-5-1964 to https://mysite.com/#1 and the permalink of the post with start_date 3-5-1985 to https://mysite.com/#2

I think I need to count all the posts, order by start_date, find out where the current post is within that array, and construct a url for the link. Hope this makes sense!