The true time of Fajr refers to the appearance of the second dawn according to consensus. That is indicated by the reports of the times of prayer. The second dawn is the white line that appears along the horizon; it is called the true dawn (Al-Fajr As-Sadiq).

It is not permissible to delay the prayer beyond its time because of work as work is not an excuse for delaying prayer. Zuhr and ‘Asr and Maghrib and ‘Isha prayers are the ones that can be combined (in some cases). But you cannot join `Asr and Maghrib. So you must organize your time at work in a way that will not interfere with your performing the prayers on time.