It is also established that Abu Darda, Abu Talha, Abu Hurairah, Ibn Abbaas and other companions practised this in relation to the voluntary fasting.

Performance of an obligatory action is dependant on the ability to do it.
Whether or not one ate or drank in the beginning of the day, if one becomes aware that Ramadaan has started, then one must immediately stop eating, drinking and all the other acts that break the fast. In this case one does not have to make up that day.

Establishing the intention (before the Fajr prayer) is not required in this case because one did not have the ability to do so. This is based on a principle of Fiqh (Jurisprudence) that: "in order for something to be conditional on the legally capable Muslim then he must have the abililty to do it"

Aa'ishah (R) said "The Messenger of Allaah (S) commanded (the people) to fast the Day of Aashooraa (10th Day of Muharram). However, when Ramadhaan was made obligatory, then whoever wanted to fast, fasted and whoever did not want to did not" [Bukhaaree and Muslim]

It is narrated from Salamah bin al-Akwaa that he said: "The Messenger (S) commanded a man from Aslam to inform the people that whoever ate must fast the remainder of the day, and whoever did not eat let him fast because this is the day of Aashooraa (the 10th Day of Muharram)" [Bukhaaree and Muslim]

It must be noted that the Messenger (S) commanded this for a fast that was obligatory and he did not make it obligatory on them to fast another day because they did not have the intention to begin the fast. Ramadaan is also obligatory and so the same ruling will apply if the month begins and one does not know. As soon as he becomes aware, he must leave off everything and fast, and it will be accepted. And Allaah knows best.

The Duration of Fasting

Beginning of fasting
Allaah (T) said:

{..And eat and drink until the white thread is distinct from the dark thread of Fajr} [Baqarah 2:187]

Adiyy ibn Haatim narrates: "When the above aayah was revealed, I placed a black rope and a white rope underneath my pillow. I continued to look at them during the night but could not distinguish between them. I, therefore, hastened to the Messenger (S) and mentioned this to him, so he (S) said: "Verily that (aayah) refers to the blackness of the night and the whiteness of the day" [Bukhaaree and Muslim]

Narrated Sahl ibn Sa'ad (R) that he said: "After this aayah was revealed {... And eat and drink until the black thread becomes distinct from white thread}, when someone wanted to fast he would tie to his feet a white thread and a black thread and he would continue to eat and drink until he could see them distinctly, so Allaah (T) revealed after that {... of the Fajr (prayer)}. Then they became aware that it meant the night and the day."

Notes:-

1. It must be known that there are two Fajrs (dawns);

a) a false Fajr: It is not allowed to pray the Fajr prayer at this time and eating is allowed for the person who intends to fast that day.

b) a true Fajr: At this time for the person who wants to fast, eating becomes prohibited so he has to stop eating and it becomes allowed to pray the Fajr prayer.

This is explained in the hadeeth of the Messenger (S). Ibn Abbaas said that the Messenger of Allaah (S) said: "There are two Fajr (dawns). As for the first, it does not prevent eating (for the fasting person) nor does it permit prayer (the Fajr prayer). As for the other it prohibits eating and allows the prayer" [Ibn Khuzaimah and Haakim]

End of Fasting
When the night comes from the direction of the east and recedes from the direction of the west and the sun sets, it is time to break the fast.

Umar said that the Messenger of Allaah (S) said: "If the night approaches from hither and the day recedes from there and the sun sets then the fasting person must break his fast" [Bukhaaree and Muslim]

The command that is established relative to this issue is that the fast can be broken directly after the last part of the sun disappears, even if light can still be seen.