Time crystals are neat! A normal crystal is one that has a definite repeating pattern in space. A time crystal is similar, but instead of repeating in space, it repeats in time.

https://en.wikipedia.org/wiki/Time_crystal

Here's a gratutious example thanks to ChatGPT:

"Imagine a line of atoms that flip their spins periodically, creating a pattern that repeats not only in space (like the atoms are arranged in a regular grid) but also in time (like the atoms flip their spins at regular intervals). This repeating pattern in time, which occurs without continuously adding energy, is what makes time crystals so intriguing and different from other states of matter."

FWIW, and IANAP, a time crystal is a structure that causes things to apparently reverse their structure back and forth without additional energy. This is similar to a spatial crystal in that the physical configuration repeats in space, except the configuration repeats in time without additional energy.

A time mirror causes a configuration to reverse in a way that’s a reflection of the original. Someone in a parallel thread gives examples of this.

They’re not the same thing.

IMO time crystals are more amazing.

Someone should make a time-image about time-crystals.