Hi Pornthep
If I understand your question correctly, you have bought a Half-Fare travelcard valid from 20 March. In this case, you can travel at half price from this date on and not on the day before.
For any further questions regarding your SwissPass or your Half-Fare travelcard subscription, please do not hesitate to contact our Rail Service on +41 848 44 66 88 (0.08/min.) or +41 51 225 78 00.
Have a nice day. EllenvS.