I say, with God's guidance: Prayer is obligatory at any part of the time absolutely, and it is up to the responsible person to designate it by performing it. However, if he does not pray until the time becomes tight, that part becomes obligatory, and if he delays it beyond that, he sins; because God Almighty commanded prayer at any time, so it is not restricted to a specific part. See: Al-Ikhtiyar 1: 51, and God knows best.