How do i make a login bonus system?

What you need to do is use os.time() with DataStores. When players leave, save a key with the os.time() and when players enter then check if there's any data saved. If there is then compare the saved data with the current os.time() to check time differentiation.

Why should you use os.time rather than tick? Because tick() is localized to the client's console, meaning that the time can be altered by either the client logging on from a different time zone OR changing the system's time settings and exploiting your daily award system. os.time returns a time that is same between all consoles, so these little methods are no longer possible.

Example;

local ds = game:GetService('DataStoreService'):GetDataStore('DailyRewards')
local dayMin = 60*60*24
local dayMax = 60*60*24*2
--The daily time areas.. Between 24 hours and 42

game.Players.PlayerAdded:connect(function(plr)
    local key = tostring(plr.userId).."_time"
    local data = (os.time() - ds:GetAsync(key)) or 0
    if data >= dayMin and data <= dayMax then
        print('Award player here')
    end
end

game.Players.PlayerRemoving:connect(function(plr)
    local key = tostring(plr.userId).."_time"
    local data = ds:GetAsync(key)
    if data then
        ds:UpdateAsync(key,function(old) return os.time() end)
    else
        ds:SetAsync(key,os.time())
    end
end)

I also suggest looking into game.OnClose in order for safe game crashes and the ability to shutdown all servers when you want to update.