I say, and with Allah's success: It is permissible to perform tayammum before and after the time of prayer; because tayammum is a substitute for wudu (ablution) and ghusl (full ablution); as narrated by Abu Dharr  that the Prophet  said: "Indeed, pure soil is the ablution of a Muslim for ten years, and when water is found, let him touch his skin with water," in Sahih Ibn Hibban 4: 139, Musannaf Ibn Abi Shaybah 1: 144, Musnad Ahmad 5: 146, Sunan Al-Daraqutni 1: 187, Sunan Al-Bayhaqi Al-Kubra 1: 187, and Ibn Al-Qattan authenticated it. Refer to: Nasb Al-Rayah 1: 148, Al-Darayah 1: 67, Khulasat Al-Badr 1: 70, and Al-Wiqayah p. 110. However, it is preferable to perform tayammum after the time has entered; to avoid the disagreement among the scholars, and Allah knows best.