In this paper, we investigate simultaneous wireless information and power transfer (SWIPT) where a multi- antenna transmitter sends data and energy to single antenna receivers with a time switching (TS) circuit. In this system, a general joint TS protocol is introduced which includes conventional TS schemes as special cases. We aim to analyze the achievable rate region of the joint TS under energy harvesting constraint at the receivers by jointly optimizing the TS ratios and the transmit covariance matrices. To tackle non-convex rate region characterization problems, we first decouple the original problems into several subproblems with fixed auxiliary variables. Then, the globally optimal TS ratios and the transmit covariance matrices are computed via convex optimization techniques. Numerical examples verify the efficacy of the proposed joint TS over conventional methods.