All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The time of the ‘Ishaa' prayer begins when the red twilight disappears from the sky. ‘Adullaah ibn ‘Amr ibn Al-‘Aas reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: "The time for Maghrib lasts until the twilight has faded and the time for ‘Ishaa’ lasts until midnight.” [Muslim]
Abu Hurayrah reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: "The time for Maghrib starts with sunset and lasts until the twilight has faded. The time for ‘Ishaa’ starts when the twilight has faded and lasts until midnight." [At-Tirmithi]
The time interval between Maghrib and ‘Ishaa' differs according to various seasons. It may be long or short, approximately an hour and fifteen minutes to an hour and thirty-five minutes.
The criterion here is the visual sighting. Whenever the red twilight fades away, the time of Maghrib elapses and the ‘Ishaa' prayer becomes due. There is no harm in adopting specific prayer times as long as you are sure of their accuracy.
Moreover, there is no harm in adopting Al-Aqsa prayer timings if they are proven to be accurate and as long as your city and Al-Aqsa share the same sunset time. You should not calculate the ‘Ishaa' prayer's time by simply performing the prayer after the passage of an hour and fifteen minutes after the Maghrib prayer all year long. The time gap between the Maghrib and ‘Ishaa' times differs for various seasons, as stated above.
At the same time, Muslims should be keen to foster unity, brotherly love exchange sincere advice and denounce blindly following the habits if they contradict the Islamic Sharee‘ah and incur nurturing discord and disunity among Muslims.
It is unbecoming of Muslims, especially if they live in one country, to raise the Athaan for the same prayer in the city's mosques with such a long time interval (i.e. 20-25 minutes)!
This incurs confusion and inconvenience, especially for women and those who are exempted from performing the congregational prayer at the mosque (due to a valid Sharee`ah-acceptable excuse).
Instead, they should all agree on one time to proclaim the ‘Ishaa' Athaan in all the mosques of the city; let it be at the beginning time of ‘Ishaa' so as to follow the Sunnah of the Prophet, sallallaahu ‘alayhi wa sallam so as to avail of the most primary function of the Athaan (i.e. reminding people that the prayer is due).
Ibn Qudaamah said in Al-Mughni, "It is recommended to proclaim the Athaan at the prayer's beginning time in order to remind people of its due time so that they can get ready to perform the prayer. Jaabir ibn Samurah said that Bilaal never delayed the Athaan (from the prayers' beginning times).”
Hence, competent scholars and experts should be consulted to verify that Al-Aqsa prayer times are accurate. If they were found accurate and corresponded with your city's prayer times (and the sunset time in Al-Aqsa is identical to that of your city), they should be set as standard and be adopted by all the mosques in the city. If not, then each Mu’ath-thin should strive to make the visual sighting of the red twilight as it fades away or adopt approximate prayer times.
Allaah Knows best.