Every proponent of natural law will reject the title above: natural law is ancient (that is when it arose) and timeless (that is how it applies).

I entirely agree, if natural law is understood as the source of universal promptings of conscience. I agree with Erasmus:

The Law of Nature, carved deeply into the minds of all, tells Scythians as well as Greeks that it is unjust to do to another what one does not wish to suffer himself.  

Desiderius Erasmus, A Diatribe Concerning Free Will (1524)

There is a law woven into reality that is manifest in us, as

what we can’t not know.

J. Budziszewski, What We Can’t Not Know: A Guide,
2nd ed. (San Francisco: Ignatius, 2011)

Agreed.

Agreed, in fact, among groups who tend to be seen as reticent about natural law. You may have noted, for instance,

the contemporary ambivalence among many Orthodox (if not hostility) toward natural law…. 

Dylan Pahman, “Fr. Michael Butler: Orthodoxy and Natural Law,”
Acton Institute Powerblog (21 June 2013)

Or the attitude of Protestants:

it is eminently fair to ask why Protestants have been so critical of the natural law down to the present day, and vehemently so.

Daryl Charles, “Reforming Natural Law,” First Things (January 2007)

But it is simply incorrect to reject natural law. It is perfectly demonstrable that natural law, as described above,  

is present in the Eastern Tradition from the ancient to the medieval and modern periods….

Fr. Michael Butler summarized by Pahman

The same goes for Protestantism. (Let this stand for now; support for some of this to follow tomorrow.)

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But here is the question: why are we hearing about natural law today?

I have taught natural law because it is important (“Know thyself”) but its importance for our time is not usually identified with that. The problem today is our moral/political differences, on which self knowledge is impotent (we are told) ‒ but natural law is another story.

The dominant reason for talk about natural law today appears to be that natural law can

provide common moral and even political grounds for people who do not agree on the existence or the nature of God and the role of God in human affairs.

Despite the depth of our differences,

hope exists

for arriving together at the conclusions delivered by natural law arguments ‒

because we really do possess the capacities for reasonableness and virtue; truth ‒ including moral truth ‒ is accessible to us….

Robert George, “Natural Law, God, and Human Dignity,” 
The Chautauqua Journal 1:8 (2016), 8, 10

This is where we see the Enlightenment version of natural law, which this is.

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Augustine did not agree (or so I suggest), because he was properly Pauline (I’ll make the case at some point and return here to add a link).

Protestants with the same Pauline roots will also not agree (one might be a little bolder and say Protestants who are Christian will not agree, but without proper argument that might seem rude) (by the same token neither should Catholics). Nor will the Eastern Orthodox agree ‒ and here is where, I think, a good deal of that “ambivalence” and “vehement opposition” is centred.

Allow me merely to register that opinion, so as to get on to the main point here, which is that there is a difference between the assertion of natural law ‘truths’ (shared by the ancients, Catholics, Orthodox, and Protestants) and the formulation of natural law arguments.

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The former can be used upon anyone. The truths of the natural law have a hold precisely because they are, in a certain way, undeniable. You can overrule the voice that says

it is unjust to do to another what one does not wish to suffer himself

but that voice is there. You can ignore it but it is speaking to you from within, and it is truly hard to silence. (Thus the claim about what you can’t not know.)

But the natural law argument ‒ take as an example the argument for the nature of marriage ‒ has no hold whatsoever, unless you have accepted some further idea (far more resistible), like the idea that reality is divinely ordered (the ancients) or the world is created for man by God.

The hold of the natural law itself depends on no such view, rather on the way we are made: if we are human beings the law is in us and it can be awakened.

But the moment you attempt to construct an argument supposedly crafted of one natural-law claim set upon another, to reach a conclusion about the nature of something like marriage, you will not be doing what you imagine you are doing. You will be drawing upon something that is not a piece of human nature.

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Natural law arguments such as those developed by Thomas Aquinas are arguments for the Church. (Know thyself as a part of the Church; think the thoughts of Christ.) The natural-law case for the nature of marriage impresses me because of something I accept about what marriage is. But that is not to say (as liberals have claimed) that people like the authors of What Is Marriage?: Man and Woman: A Defense? are rationalizing (fabricating justifications for an answer they began with). These arguments unfold for ‘us’ an understanding about marriage that was not at all clear to begin with.

What they do not do is lead an outside person, resistant to some understanding of things accepted by ‘us’, to embrace what they reject. From the standpoint of the person who has no belief in, say, metaphysical order … or who scoffs at the centrality of the mated pair male-and-female, husband-and-wife (why should that be paradigm?) … to that person the argument is laughable. That is, it cannot be understood, it is babble. From that outsider’s vantage point the key moves of the natural law argument look utterly arbitrary.

If you want a real sense of that outsider view (and you owe yourself this, you ought to make the effort to do this at least once) listen in on these people who believe in reason and argument and who have given a pretty fair listen to the natural-law case for the nature of marriage. Give them as much of a hearing as they gave to natural law.

“On Robert P. George’s Natural Law Argument Against Same-Sex Marriage,”
Doubtcast (17 May 2014)

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Is this view that I have presented ignorant about the true nature of natural law? Don’t listen to me. This idea of an ‘Enlightenment version of natural law’ you can get from central figures like J. Budziszewski.

In a sketch of the history of natural law thinking he marks several phases, the third of which

was dominated by the thinkers of the Enlightenment. For various reasons … they tried to sever the connections between religion and philosophy, between faith and reason. Their aim was to make natural law theory theologically neutral ‒ a body of axioms and theorems that any intelligent, informed mind would consider obvious once they were properly presented, in fact equally obvious no matter what religion or wisdom tradition the mind followed, or whether it followed any at all.

Budziszewski, xi

To my ears that recalls nothing so much as Alasdair MacIntyre’s claim that

The thinkers of the Enlightenment insisted upon a particular type of view of truth and ra­tionality, one in which truth is guaranteed by rational method and ra­tional method appeals to principles undeniable by any fully reflective rational person.

Alasdair MacIntyre, Whose Justice? Which Rationality?
(Notre Dame: University of Notre Dame, 1988), 353

Writes Prof. Budziszewski,

The new breed of natural law thinkers … reject the fallacy that natural law is like mathematics. To see it well, one must have pure eyes, and this requires moral virtue.

Back to Augustine, St. Paul, the Church Fathers, Aquinas, the Reformers, none of whom is modern!

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Here is the important point. We cannot make the progress that we could make until we cease to be modern and give up what MacIntyre calls

the Superstition of Modernity.

Alasdair MacIntyre, Three Rival Versions of Moral Enquiry
(London: Duckworth, 1990), 170

Whatever we may think, we are not turning to natural law when we wheel in, with a Natural Law label stuck to it, the very idol of Reason that Enlightenment modernists bowed to. To believe, as they did, that on

questions of standards, criteria, and method all rational persons can resolve their disagreements

MacIntyre, Three Rival Versions of Moral Enquiry, 170

in matters of society and morality is to have turned away from the very tradition we (poorly schooled in that tradition) imagine we are returning to.