Countdown widget

Add the following in your rc.lua:

local countdown = {
    widget   = wibox.widget.textbox(),
    checkbox = wibox.widget {
        checked      = false,
        check_color  = beautiful.fg_focus,  -- customize
        border_color = beautiful.fg_normal, -- customize
        border_width = 2,                   -- customize
        shape        = gears.shape.circle,
        widget       = wibox.widget.checkbox
    }
}

function countdown.set()
    awful.prompt.run {
        prompt       = "Countdown minutes: ", -- floats accepted
        textbox      = awful.screen.focused().mypromptbox.widget,
        exe_callback = function(timeout)
            countdown.seconds = tonumber(timeout)
            if not countdown.seconds then return end
            countdown.checkbox.checked = false
            countdown.minute_t = countdown.seconds > 1 and "minutes" or "minute"
            countdown.seconds = countdown.seconds * 60
            countdown.timer = gears.timer({ timeout = 1 })
            countdown.timer:connect_signal("timeout", function()
                if countdown.seconds > 0 then
                    local minutes = math.floor(countdown.seconds / 60)
                    local seconds = math.fmod(countdown.seconds, 60)
                    countdown.widget:set_markup(string.format("%d:%02d", minutes, seconds))
                    countdown.seconds = countdown.seconds - 1
                else
                    naughty.notify({
                        title = "Countdown",
                        text  = string.format("%s %s timeout", timeout, countdown.minute_t)
                    })
                    countdown.widget:set_markup("")
                    countdown.checkbox.checked = true
                    countdown.timer:stop()
                end
            end)
            countdown.timer:start()
        end
    }
end

countdown.checkbox:buttons(awful.util.table.join(
    awful.button({}, 1, function() countdown.set() end), -- left click
    awful.button({}, 3, function() -- right click
        if countdown.timer and countdown.timer.started then
            countdown.widget:set_markup("")
            countdown.checkbox.checked = false
            countdown.timer:stop()
            naughty.notify({ title = "Countdown", text  = "Timer stopped" })
        end
    end)
))

then, add countdown.widget and countdown.checkbox to your favourite wibox.

Left clicking on the checkbox will prompt for a countdown, right clicking will stop the timer.

The widget will display the countdown.