Question Summary:
Time for sunset and iftar

Question Detail:

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Assalamu Alaikum,
For example, in Chittagong, Sunset is at 5.11 pm .....
When should I break my fast? at 5.12 pm or 5.13 pm?
how many minutes should be in gap between sunset and iftar ?
What timing would be preferable
​/​
permitted in this timing cases?
please let me know clearly.....
In my area, one masajid whistles after 2 minutes, another masajid whistles after 3 minutes of Sun-set . Should I follow the whistle of my nearest Masajid or I should go according to my own calculations/timings gathered?
​I mainly follow the below website to calculate/gather Timings:https://www.timeanddate.com/astronomy/bangladesh/chittagong

Jazakallahu Khairan .
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Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
The time for iftaar is at sunset. Azan is not necessary to break the fast.[1]
The Fuqahaa have stated that it is Waajib to exercise precaution in breaking fast.[2]
The computed timetables for sunset are generally accurate. A delay of two or three minutes after the sunset time is acceptable.
If your local masjid has sought the council of an expert, then it would be correct for you to follow the whistle of your local masjid. 
And Allah Ta’āla Knows Best 
Saad Haque
Student Darul Iftaa
New Jersey, USA
Checked and Approved by,
Mufti Ebrahim Desai.