All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
What we understand from the question is that the imaam delays the athaan by 20 minutes, not that he advances it, as you said. For example, if the athaan, according to the calendar, is at 5:00 a.m., he calls it at 5:20 a.m. If this is the case, then we have the view that the imaam is not permitted to do so; rather, he should look for the most trusted calendar and the most correct one and rely upon it.
He should not delay the athaan from the time that is indicated in the calendar so that he would not mislead the people who are fasting and not make people miss the virtue of praying at the earliest time of the prayer.
Shaykh Ibn 'Uthaymeen said:
“It is not permissible for the mu'aththin to deliberately delay the athaan half an hour after the beginning of the time of the prayer, because if he does so, he makes the people miss the virtue of praying at the earliest time of the prayer, and perhaps the athaan is for the Fajr prayer, and some people may be fasting, so they eat until his athaan, after the Fajr time has already begun [and this makes their fasting void], or they delay breaking their fast if this happened at the Maghrib athaan, until he calls the athaan a while after sunset.”
If there are different calendars, then you should act upon the most trustworthy one and the one that is closest to the truth in your opinion, and this will be sufficient for you – Allah willing.
Allah knows best.