[circonian]

There are a bunch of functions for getting time in the game. Look at the API under the global section: http://wiki.esoui.com/API#Global

I didn't test any of this -- But it should point you in the right direction

You could call a function on intialize to get the time you start playing. Needed to check and see if you started playing before 3am or after 3am.

-- call it on initialize & save the time you start playing --
local function LogTime()
    -- Capture start playing time --
    myAddon.StartTime = GetSecondsSinceMidnight()
end

You could then use the following and register it to some event that happens frequently so its checked often.

function myAddon.TimeAlert()
    local iSecSinceMidnight = GetSecondsSinceMidnight()
 
    -- 3 am = 3 hours past midnight, 3 hours * 60 min/hour * 60 sec/min = 10800 seconds --
    
    -- If you start playing at 10pm we dont want the alarm going off yet, so we check the start time: --
    -- (If you started playing After 3am) and (its earlier than you started playing, then its the next day) and (its after 3am now) --
    -- or (you started playing before 3am) and (its now after 3am)
    if (((myAddon.StartTime > 10800) and (iSecSinceMidnight < myAddon.StartTime) and (iSecSinceMidnight > 10800) ) or
    ((myAddon.StartTime < 10800) and (iSecSinceMidnight > 10800))) then
        -- do whatever you want your alarm to be --
 
        -- Create a window to pop-up, unhide it at 3am --
        myAddon.popUpWindow:SetHidden(false)
        
        -- Display a chat Message --
        d("It's 3 A.M.")
        
        -- Play some sound:  5 times, once every second to make sure you here it --
        for i = 1, 5 do
            zo_callLater(function() PlaySound(SOUNDS.GENERAL_ALERT_ERROR) end, (i*1000))
        end
    end
end

You could also do what was suggested & take that same information & do a timer. I've never messed with anything like that except timing something for a few seconds. I don't know how accurate a zo_callLater would be for an extended period of time or if it has a time limit on it (it may not work if set for several hours I have no clue). Theres probably a better way to set a timer, but the rest of it should point you in the right direction to get the information you need to set a timer.

-- call it on initialize & get the time you start playing --
function myAddon.LogTime()
    -- Capture start playing time --
    local iStartTimeSecs = GetTimeStamp()
    local iAlertTime = 10800
    
    -- If you started after 3am --
    if iStartTimeSecs > 10800 then
        -- Calculate the number of seconds left in the day --
        -- 24 hours/day * 60 min/hour * 60 sec/min = 86400 sec/day --
        local iNumSecsTillMidnight = (86400 - iStartTimeSecs)
        -- and add it to the number of seconds after midnight (for your alarm) --
        iAlertTime = iAlertTime + iNumSecsTillMidnight
    end
 
    -- I have no idea how accurate this would be or if it would even work over an extended period of time --
    zo_callLater(function() myAddon.PlayAlert() end, (iAlertTime*1000))
end
 
function myAddon.PlayAlert()
    -- Do whatever, same as above --
end