Comment 3 for bug 2063340

As mentioned in comment #2 the values in the fields haven't actually become large enough for this to matter yet, so this can't be "actual" overflow; it has to be just a calling convention issue that Lisp is getting wrong.
I imagine that no active SBCL committer has access to a machine using the wider time types (i.e. afflicted with the problem) to figure it out; I certainly don't.
There should be a straightforward way to diagnose further if not repair: intercept any of the time-related calls in C and print the arguments/results in hex; similarly print them in Lisp and visually compare to figure out what's going wrong. The fix should become self-evident. I don't know who can do it for you.