manifest shadow

I've been experimenting with this spell a bit, and it doesn't seem to work like the helpfile says it should. The helpfile says the shadow will guard the site of its summoning for a week, unless dispelled by natural daylight. I understand that therefore it gets dispelling in Galadon's graveyard. However, there are graveyards deep underground, some too dark to scan, and yet the shadow still doesn't last a week. The timer does, but the shadow doesn't.