oh hmmm since I saw someone's post that undead spawn at day time too... so I guess I was wrong up there.
undead can be spawn day or night :O

but ya... I'm sorry but your timer is not that good because every 3 game hours it pop or depop. in the other words, it moves every 3 hours and it can be happened in multiple camps at same times...
For the undead, at least for 2 events, I waited long time until 18:00, another day was 0:00 then undead spawn. Both me and my mule see the undead spawn at 2 different camp for these 2 days of camping.

You know... sometimes I hate ppl tell me just go read more posts, too... but I donno...