Another word
ilm
The answer is so simple given by the Prophet (s) himself
The Prophet (peace and blessings of Allaah be upon him) said: ?Fast when you see it (the new moon) and break your fast when you see it, and if it is cloudy then complete the number (of days ? i.e., assume the month is thirty days).? (Saheeh Bukhari and Saheeh Muslim).
Salaam