Timelessness

You can carbon date an interface. Stitched leather means 2011. Long shadows mean 2014. Frosted glass means 2021. Every look that arrives as a wave leaves as one.

Here is one button wearing four eras. The shape, size and label never change.

Only the surface moves, and the surface is what gives the date away. Surface ages fast. Structure ages slowly.

Long-lasting

One of Dieter Rams's ten principles is that good design is long-lasting, and his definition is blunt: "It avoids being fashionable and therefore never appears antiquated." The shelving system he designed for Vitsœ in 1960 is still sold today.

Paul Graham makes the same point from the other side. Fashions change by definition, so anything that still looks good far in the future must be getting its appeal from merit. Shu Ding puts it as wanting to "disentangle time" from his design process.

What ages well

Structure. Hierarchy, spacing, alignment and rhythm read the same in any decade. Switch the surface and watch which card survives.

A dated surface makes the right card look old. It never makes it hard to read. The left card is hard to read in every era, including this one.

Function. Choices with a reason behind them, like a visible focus ring or a hit area that fits a fingertip, don't date, because the reason doesn't.

Restraint. A monochrome, type-led interface is very hard to date. That is a big part of why terminal aesthetics keep coming back.

Not a ban on style

Timelessness isn't beige. It is one question, asked before adopting anything: would this still make sense if the trend around it vanished tomorrow?

A trend is a good reason to examine a technique. It is never a reason to use one.