This may seem like an obvious statement, but it's less obvious than you might think: It's tougher to write something timeless than something for the here and now. This may seem obvious, and it's hard enough to write something for the here and now, but it's an infinity number of times to write something timeless because timeless art has to seem like it's of the here and now every time it's viewed.
Nothing dates like the cutting edge. There is no guarantee that creating art in an older style will be of any more worth than creating in a newer one. It's also true that most creators who are obsessed by style are obsessed by finding new styles in which to create rather than old ones, and there is, sadly, a near-guarantee that if a creator's fundamental concern is the style in which he writes, he will create something of no worth outside of its style. By definition, previous eras are where we can find the preponderance of eternal art. Posterity has not yet subjected our own era to its merciless tests. We can guess as to who or what makes the eternal work of our era, but we have no way of judging properly. The only people who can judge what's worthwhile to posterity are people who stand outside the cultural milieu in which the work's created. The closer we are to the era, the background, the epoch, the more bias we have in judging its value. Furthermore, an artist who is only interested in the new will have ignored millennia's worth of quality work, and while his work would suffer enormously if he neglects to embrace the new, it will suffer exponentially more enormously if he neglects to embrace the old. If you want to understand what it truly means to create great art, look to older models.
Frank Martin's Mass for Double Choir was written in the early 1920's, and it sounds like the music of an ethnically French composer who was heavily influenced by Debussy and Ravel. Yet at the same time, it also sounds like it could just as easily have been written 350 years earlier by Palestrina or Lassus or Victoria. It creates its effect by fusing the styles of two eras to a seamless extent that only a genius can.
In the early 20th century, Switzerland had three composers who remain sleeping giants among music lovers. One, Artur Honegger, gets at least a bit of the recognition he deserved, though nowhere near enough. Another, Ernest Bloch, used to get a bit of the recognition he deserved, but even at his most famous, it was nowhere near enough. The third, Frank Martin, never got anywhere near the recognition he deserved for anything but this piece.
Martin wrote like a man of the sixteenth century because, in many ways, he was one: a Swiss from Geneva, descended from Huguenots, and the always devout son of a Calvinist pastor. He was perfectly aware of the great developments of music in the countries surrounding him, and being Swiss, it was particularly easy for him to move in the worlds of French, German, and Italian music - living as he did at different times in Zurich, Paris, Rome, and retiring to Amsterdam. He became, at different points, interested in Bulgarian music, in Hindi music, even in jazz. He was, however, formed in the timelessness European landscape which perhaps only a Swiss youth could enjoy in the early 20th century. His formative influences were Bach, and the even earlier music of the Lutheran hymnal. He was fully a composer of the 20th century, perhaps even of the 21st, but the greatness of his music lies in the tension between the broad outside world of which he was very much aware, and pastoral, timeless environment in which he was formed.