Marras
 

I know it's an adverb meaning long ago, but I am confused about how it's used.
For instance would you say: Lo hice marras:thinking:

That use is very outdated, so I couldn't give you a living example.

Much probably, the use of marras in set phrases meaning "lo de siempre" or "algo de vieja data" (the same old thing/story/****) would lead to the actual use of that word in the set expression de marras, which is an adjectival phrase meaning "something that is known by everyone" or "something which details or intricacies are widely known:

Vino el individuo de marras (You know who, came) [sometimes used with magical thinking: deliberately not named]

Sigo con el problema de marras (I can't solve that problem I told you extensively about)

Also survives in the colloquial noun phrase "lo de marras", meaning "what you and me know but prefer to refer indirectly", often used jocularly.

I agree with Alec, except that in Spain "de marras" is still very much in use and very current, if you wish.

DRAE gives,

de marras
1. loc. adj. Que es conocido sobradamente. Ha contado mil veces la aventura de marras. Vino a verte el individuo de marras.

I personally never heard "marras" in the sense of "days gone by", which per DRAE is actually outdated.

Ejemplo del CREA:
Como se dice que el pueblo chileno es mayoritariamente católico y el canal de televisión [***]de marras[***] pertenece a una universidad de la Iglesia católica,

AÑO: 1996
AUTOR: PRENSA
TÍTULO: La Época, 27/06/1996 : Maltrato infantil
PAÍS: CHILE
TEMA: 02.Testimonios varios
PUBLICACIÓN: (Santiago), 1996

¡Saludos cordiales!

In Argentina too, as it is everywhere.

I meant the adverb marras, meaning antañamente*, is what is dated.

Atales avié d'ellos qe metién apellidos,
qe los oviera marras en Cogolla vencidos;
si de tan rehez omne fincassen escarnidos,
mucho máes lis valiera qe non fuessen nacidos.

The Life of Saint Millán of La Cogolla
Gonzalo de Berceo
circa 1230
[Now you can understand what I experience "perusing" (glancing through) Chaucer's Canterbury Tales]

OK de marras is like the very contemporary from back in the day.or de antaño or of yore.

By the way, Chaucer's English is in no way understood by modern speakers without help. It's Shakespeare's English which is similar to modern, but requires attention.

No. You have to forget the definition you asked in post #1. There's no modern -not even three centuries old- use of marras that matches it with antaño.

De marras is a very important expression that doesn't relate with time.

Thanks for clarifying that.

Entendido, Alec... :)