Peace be upon you and God's mercy and blessings.
Ayatollah Khamenei (may God protect him):
The basis for the time of the evening prayers and iftar is the sunset, specifically the disappearance of the eastern redness, and it cannot be changed to that of other jurists.
Ayatollah Sistani (may God protect him):
The time for iftar is when the eastern redness disappears, even for those who are aware of the sun's setting, as a precautionary measure.
Thus, the individual has two options: either to act with caution or to refer to another authority on this matter, keeping in mind the most knowledgeable among them.
Source: Official website: 1, 2. May you be successful in all good.