Recently, it has been proposed that various kinds of time operations can be performed using an excitable field, mainly based on computer simulation. In this study, we performed experiments toward the realization of a time operation, such as time-difference detection. We used the photosensitive Belousov-Zhabotinsky reaction as a spatially distributed excitable field. We found that a single geometrical circuit can perform different operations with changes in the intensity of light illumination. The experimental results are discussed in relation to the idea of a non-Neumann-type computational device. (C) 2003 American Institute of Physics.