Ignore tombstone values completely in days_since_latest

Given that tombstones are impossible for sets I guess this doesn't necessarily "fix" anything.  But it brings the "days_since" functions inline with each other. The "days_since_*_by" functions were already doing this inadvertently.
Just to confirm/double-check - is this OK? This data is currently/basically states, so I guess it makes no sense to run "days_since_*" on it?