After a week of binge-hunting timed-popped and lotto-popped NMs, I noticed a pattern of when mobs would repop. Apologies in advance if this is already common knowledge, but I couldn't find any posts that mention this.
Using the time according to my clock in Win7, mobs will only repop at :00seconds or at :30seconds, after their respawn cooldown has passed. For example, if a mob with a 5min respawn dies at 5:00:01, it's next repawn time would be 5:05:01, but it actually will not repawn until 5:05:30. If the same mob dies at 5:00:29, it will also respawn at 5:05:30. If it died at 5:00:31, its next respawn would be 5:05:31, but it actually will not respawn in 5:06:00. Note that the seconds they pop may vary person to person depending on if/where your clock is synchronized too. However, there should only be two times a minute when a mob can pop, and these times are 30seconds apart (i.e. for someone else they may pop at :23 and :53).
I measured the ToD as the time you get the message "<mob> was defeated by <player>". I've tested this on over 300 normal mobs in Zilart, CoP, and WotG areas. I've also tested a few notorious monsters like Hyakinthos, Lamina, Goblin Trap, and a few others, for over 100 spawns. For Hyakinthos and Lamina, I made the assumption that they could pop every 5 mins(since that is the respawn time for other Lycopodium in those areas) between 90mins and 120mins after they last died, which was the case every time I was there. In every case, the mobs and NMs would spawn within about 1second of the :00 or :30 mark(probably slightly delayed because of lag). I also made sure to kill a mob at every possible second to make sure that the time I was taking as the ToD was correct.
I do not have any experience with HNMs such as zilart kings or any 21-24hr pop NMs. HOWEVER I was told that this it NOT how it works for kings. Instead, I'm told that for zilart kings, the ToD is measured as the time the body starts to fade away. They will also respawn at the same second as they started to fade, i.e. if its body starts to fade at 8:00:14, it's first chance to repop is 5:00:14 the next day.
Can anyone else confirm my findings with normal mobs/NMs? And am I correctly informed about how zilart kings pop? If both of these are correct, then that also asks the question, which mobs follow which spawn timing?