How accurate are the calendars that determine prayer times, especially Fajr and Maghrib, and are there differences between them and the actual reality?

Prayer times are known by cosmic signs, and calendars approved by trustworthy scholars are relied upon unless they contradict the Shariah signs. As for unknown calendars, they are not relied upon entirely, and one should act based on certainty or strong probability. Fasting is linked to the true dawn and sunset. So, whoever eats the pre-dawn meal (suhoor) while doubting the arrival of dawn, their fast is valid. But whoever breaks their fast while doubting the sunset, their fast is invalidated. One should be cautious regarding abstaining (imsak) and breaking the fast (iftar), by refraining from eating and drinking at dawn according to the calendar, and delaying the prayer until dawn becomes clear.