[Phanx]

Well, since the argument passed right after the duration is the time the aura expires, getting the time remaining is trivial:

element.CustomFilter = function(self, unit, iconFrame, name, rank, icon, count, debuffType, duration, expirationTime, caster, isStealable, shouldConsolidate, spellID, canApplyAura, isBossDebuff, isCastByPlayer, value1, value2, value3)
	local timeLeft = expirationTime - GetTime()
	return timeLeft < 60
end