When is C-A?

C and a can't be used to express time.

A and C are heavenly stems, and there is no fixed representative time. The representative time is earthly branch, from 23: 00 to 1, 1 to 3: 00, from 3: 00 to 5: 00 and from 5: 00 to 7: 00.

In the traditional calendar of China, A, B, C, D, E, Ji, G, Xin, Ren and Gui are called "Top Ten Heavenly Stems", while Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai are called "Twelve Earthly Branches".