Raid lockout

Raid timers are the technique implemented by Blizzard to prevent members of large guilds from visiting high-end instances over and over to repeatedly farm bosses in a short period of time. Despite the name, a similar system also affects heroic dungeons.
Players re-entering an instance will find that bosses killed in previous visits to the same instance stay killed until the timer resets. The other implication is that the players associated with the instance have a time limit in which to complete their activities before the instance is reset. This time can be extended (from patch 3.2.0) with the Instance Lock Extension feature. 
Raid timers (or Raid IDs) are the system used to save you to a raid instance. To see which raids and boss encounters you are currently saved to, you can type "/raidinfo".
==Lockout Types==
===Loot-based Lockout===
Used for: All Raid Finder raids, Siege of Orgrimmar Normal/Heroic, Warlords raids Normal/Heroic.
*You can kill a boss as many times per week as you like, but only loot it once per difficulty.
*Anyone can join anyone’s instance.
*Bosses are never locked to you, and you can always go back and kill them later even if you’ve skipped them by joining a group that had progressed further.
*Raid sizes can be anywhere from 10 to 30 players, and difficulty automatically scales to match.
===Flexible Boss-based Lockout===
Used for: Normal difficulty raids from Wrath of the Lich King through Throne of Thunder.
*You can only kill a boss once per week, and you may not enter an instance where a boss is alive that you have already defeated that week.
*You can move from group to group during a week as long as you’re joining instances that don’t have bosses alive you’ve already killed.
*If you enter an instance that is more progressed than your own and get saved to it, you will be locked to all prior bosses.
*You can move between 10 and 25 raid size, given these constraints.
===Strict Instance-based Lockout===
Used for: All classic raids (Molten Core through Sunwell), Heroic difficulty of Wrath of the Lich King raids through Throne of Thunder, and all Mythic difficulty raids.
*Progress is tied to a fixed instance ID. Once tied to an instance ID, you may not enter any other versions of that raid of that difficulty until your raid lock resets.
*If other players complete that instance while you are absent, the instance will be empty for you as well, because progress is tied to the instance, and not your character.
*You may not switch between 10-player and 25-player raid size once locked to one size or the other (does not apply to Mythic which is 20-player only).
==Instance timers==
Since patch 1.9.0, instances are reset according to a fixed calendar. This was decided by Blizzard to be fairest to most and replaces the previous system of resetting raid instances based on the absence from the instance of the raid members.
Unless a raid has been extended, the following is a list of when each raid is reset:
===Classic raids===
*Molten Core: Every Tuesday at 11:59 AM
*Blackwing Lair: Every Tuesday at 11:59 AM
*Ruins of Ahn'Qiraj: Daily at 11:59 AM
*Temple of Ahn'Qiraj: Every Tuesday at 11:59 AM
===The Burning Crusade raids===
*Karazhan: Every Tuesday at 11:59 AM
*Gruul’s Lair: Every Tuesday at 11:59 AM
*Magtheridon: Every Tuesday at 11:59 AM
*Tempest Keep: The Eye: Every Tuesday at 11:59 AM
*Coilfang Reservoir: Serpentshrine Cavern: Every Tuesday at 11:59 AM
*CoT: Battle for Mount Hyjal: Every Tuesday at 11:59 AM
*Black Temple: Every Tuesday at 11:59 AM
*Sunwell Plateau:: Every Tuesday at 11:59 AM
===Wrath of the Lich King raids===
*Naxxramas: Every Tuesday at 3:00AM or during weekly maintenance
*Obsidian Sanctum: Every Tuesday at 3:00AM or during weekly maintenance
*Vault of Archavon: Every Tuesday at 3:00AM or during weekly maintenance
*Ulduar: Every Tuesday at 3:00AM or during weekly maintenance
*Trial of the Crusader: Every Tuesday at 3:00AM or during weekly maintenance
*Onyxia's Lair: Every Tuesday at 3:00AM or during weekly maintenance 
*Icecrown Citadel: Every Tuesday at 3:00AM or during weekly maintenance
===Cataclysm raids===
All Cataclysm raids reset at the same time: Tuesdays at 11:59 AM
===Heroic Dungeons===
Heroic dungeons are reset every morning at 9:00AM (7:00AM EU).
==Patch changes==
===Retail===
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*
===Classic===
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 Raid timer