Creature Spawn Inspector

GTT_CreatureInspect.lua

--
-- Creature GUID Inspector
--

function GTT_CreatureInspect(self)
    local _, unit = self:GetUnit();
    local guid    = UnitGUID(unit or "none");
    local prefix  = string.match(guid, "^([CVP][^l][^-]+)");

    if not guid or not prefix then
        return;
    end

    if string.find(prefix, "^[CV]") then
        local guidSpawnOffset  = bit.band(tonumber(string.sub(guid, -6), 16), 0x7fffff);
        local guidSpawnIndex   = bit.rshift(bit.band(tonumber(string.sub(guid, -10, -6), 16), 0xffff8), 3);
        local serverSpawnEpoch = GetServerTime() - (GetServerTime() % 2^23);
        local guidSpawnTime    = serverSpawnEpoch + guidSpawnOffset;

        if guidSpawnTime > GetServerTime() then
            -- Correction for units that survived the last epoch.
            guidSpawnTime = guidSpawnTime - ((2^23) - 1);
        end

        local guidSpawnDate = date("%Y-%m-%d %H:%M:%S", guidSpawnTime);

        GameTooltip_AddBlankLineToTooltip(self);
        GameTooltip_AddColoredDoubleLine(self, "GUID", guid, NORMAL_FONT_COLOR, HIGHLIGHT_FONT_COLOR);
        GameTooltip_AddColoredDoubleLine(self, "Spawn Date/Time", guidSpawnDate, NORMAL_FONT_COLOR, HIGHLIGHT_FONT_COLOR);
        GameTooltip_AddColoredDoubleLine(self, "Spawn Time Data", string.format("%.6X", guidSpawnTime), NORMAL_FONT_COLOR, HIGHLIGHT_FONT_COLOR);
        GameTooltip_AddColoredDoubleLine(self, "Spawn Index Data", string.format("%.5X", guidSpawnIndex), NORMAL_FONT_COLOR, HIGHLIGHT_FONT_COLOR);
    elseif prefix == "Pet" then
        local guidPetUID     = string.sub(guid, -8);
        local guidSpawnIndex = tonumber(string.sub(guid, -10, -9), 16);

        GameTooltip_AddBlankLineToTooltip(self);
        GameTooltip_AddColoredDoubleLine(self, "GUID", guid, NORMAL_FONT_COLOR, HIGHLIGHT_FONT_COLOR);
        GameTooltip_AddColoredDoubleLine(self, "Pet UID", guidPetUID, NORMAL_FONT_COLOR, HIGHLIGHT_FONT_COLOR);
        GameTooltip_AddColoredDoubleLine(self, "Spawn Index", guidSpawnIndex, NORMAL_FONT_COLOR, HIGHLIGHT_FONT_COLOR);
    end
end

if not GTT_CreatureInspectHooked then
    GTT_CreatureInspectHooked = true;
    GameTooltip:HookScript("OnTooltipSetUnit", function(...) return GTT_CreatureInspect(...); end);
end

I figured this out last year, cool to see others have too.