Most guilds that raid don't have a timezone they raid in persay, mobs die on the server upon respawns so it is mob respawns that determine when a raid target is called. Many guilds value folks that are in different time zones to provide better coverage for targets that spawn during times where server population is lower.
Since the majority of the server population is based in the US a lot of times GM events and quakes that trigger additional spawning of mobs usually happen during peak US play times when the overall server population is high.
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