[Fizzlemizz]

Your regionEventStartTime table is broken so I made what I think it might be intended to look like.

EDIT: My math foo is broken. Fixed the calculation if you logged in while the event is in progress.

As in (not very different):

local Backdrop = {
    bgFile = "Interface\\DialogFrame\\UI-DialogBox-Background",
}
 
local frame_x = 0     
local frame_y = -200    
f = CreateFrame("Button", "ZAMROTimer", UIParent, "BackdropTemplate")
f:SetWidth(255)                                          
f:SetHeight(20)
f:SetBackdrop(Backdrop)
f.text = f:CreateFontString(nil,"OVERLAY","GameTooltipText")
f.text:SetTextHeight(15)
f.text:SetPoint("CENTER")
f:SetClampedToScreen(true)
f:SetPoint("CENTER",UIParent,"CENTER",frame_x,frame_y)
f:EnableMouse(true)
f:SetMovable(true)
f:RegisterForDrag("LeftButton")
f:RegisterForClicks("AnyUp")
f:Show()
f:RegisterEvent("PLAYER_ENTERING_WORLD")
f:SetScript("OnDragStart",function(this) 
    this:StartMoving()
end)
f:SetScript("OnDragStop",function(this)  
    this:StopMovingOrSizing()
    frame_x,frame_y = this:GetCenter()
    frame_x = frame_x - GetScreenWidth() / 2
    frame_y = frame_y - GetScreenHeight() / 2
    this:ClearAllPoints()
    this:SetPoint("CENTER",UIParent,"CENTER",frame_x,frame_y)
end)
 
local function printTime(timetotrun, inevent)
    local hideSeconds = timetotrun >= 120
    local msg = "Next event is in: %s"
    if inevent then
        msg = "%s until event ends"
    end
    f.text:SetText(format(msg, SecondsToTime(timetotrun, hideSeconds)))
end
 
 
regionEventStartTime = {
    [1] = { -- eu
        starttime = 16862400102,
        eventDuration = 900,
        eventIntervalInSeconds = 5400,
        enable = true,
        datablock = {}
    },
}
local inEvent, timeToRun
local eventTime = regionEventStartTime[1].eventDuration -- Time the event runs in seconds(15 mins)
local waitTime = regionEventStartTime[1].eventIntervalInSeconds -- Time between events in seconds (90 mins)
local startTime = regionEventStartTime[1].starttime -- Start time from the table
local serverTime = GetServerTime()
local timeToEvent = (startTime - serverTime) % waitTime -- Remaining time before next event starts
 
if timeToEvent > (waitTime - eventTime) then -- Is there between 1:15 and 1:30 to go? If so, we're in the event
    inEvent = true
    timeToRun = eventTime - (waitTime - timeToEvent)
else                    -- Otherwise, set the ticker timer to time to next event
    inEvent = false
    timeToRun = timeToEvent
end
local ticker = C_Timer.NewTicker(1, function() 
    if timeToRun > 0 then
        timeToRun = timeToRun - 1
        printTime(timeToRun, inEvent)
        return
    end
    if inEvent then -- The event just finished
        inEvent = false
        timeToRun = waitTime - eventTime -- Reset ticker timer to 90 minutes wait time minus 15 mins event time
    else  -- Waiting for the next event just expired
        inEvent = true
        timeToRun = eventTime -- And the event is running
    end
    printTime(timeToRun, inEvent)
end)
printTime(timeToRun, inEvent)