The rings appear to be silver, with a Chinese ideogram of time on them. There are many symbols for time in Chinese, with different meanings in different contexts. So, the rings, if observed for a length of time, slowly change from one ideogram to another. This shift reflects the current ebb and flow around the wearer of the ring. If in the presence of a gate (even one that only transports in space, because at some level, space is time), the ring begins to shift faster, as it does at the approach of Hounds of Tindalos, or attempts at mind transferal by the Great Race of Yith.
With practice and observation, the rings can be used to gather very specific data concerning time manipulation in the vicinity. But even crude use can be helpful, using them as a sort of Geiger counter for time manipulations.
The rings can also greatly enhance time manipulations by the wearer. Gates are easier to construct and use, requiring fewer magic points. Spells manipulating time can also be easier, or more potent.