Subtracting datetimes computed each field from the signed total, so a
negative difference gave every component its own minus sign:
seconds = -95703 -> "-1 -02:-35:-3"
which does not parse back, so the interval came out as zero with the sign
still attached - "-0 00:00" where "-1 02:35" was wanted. The positive
direction was always correct, which is why this went unnoticed.
Now the magnitude is split and a single leading sign applied. Test 1635
passes.
