The original idea of time crystal is a material that has periodic structures in time, at the lowest energy state [1]. That means it can have repeated structural change over time by itself at rest. In that case, the crystal itself has an intrinsic periodicity in motion, and can be used as a source of time information.

However, now it seems to be changed to that an external energy source is driving the frequency of the structural change of the crystal over time. Like in the Yale experiment, an external pulsing light shines at the system at certain pulsing frequency and they found the system reverses its nuclear spin at twice as slow as the light's pulsing frequency. There's no intrinsic timing property shown in the crystal, so it cannot be used as a source of time information. I've asked the same question. [2]

[1] https://physics.aps.org/articles/v5/116

[2] https://news.ycombinator.com/reply?id=16997756