I just can't imagine me not having at least tried eval or eval working any differently than it is now. Perhaps I had a reason that made sense to me at the time but now I can't recall.
Addendum
I was just going over your findings. I think you get the same results whether you use set or eval and L_TIME still has to be a function of S_TIME. Maybe I did try eval after all and found no discernable advantage?
Anyhow, my trigger works as is, it would work well with how you're suggesting and I highly appreciate you looking into it. Thank you.