Every muslim should know that Nafl prayers that are linked to obligatory prayers should be performed after the beginning of their due times. In this regard we would like to quote Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto,Ontario, Canada, who stated:

There are certain recommended prayers to be done before and after certain prescribed Prayers and they are separate from the obligatory ones: Let me list them as follow:
two rak`ahs are recommended before the Fajr Prayer; two or four rak`ahs are recommended before Zhuhr and two after, and two are recommended before `Asr; two are recommended after Maghrib, and two are recommended before `Ishaa’ and two or four are recommended after Ishaa’. Then there are three rak`ahs of Witr Prayer to be performed after `Ishaa’: It can be done either immediately after the recommended rak`ahs mentioned above or later at night; but it should be done before Fajr time.