وَأَوَّلُ وَقْتِ الْعِشَاءِ إِذَا غَابَ الشَّفَقُ، وَآخِرُ وَقْتِهَا مَا لَمْ يَطْلُعِ الْفَجْرُ الثَّانِي
Translation: Isha begins when shafaq disappears and lasts until the second dawn appears.
The two interpretations of twilight may yield different start times for Isha, while the ending remains the appearance of true dawn.