Those who are confused as to the start of the month of Ramaḍān
If a Muslim is imprisoned, trapped or is in some remote location and receives no news of the time of year, it is obligatory upon him to strive his utmost in determining the start of the month.
If he does not strive his utmost to determine the time of the month of Ramaḍān, there is no difference amongst the scholars that his fast is invalid, whether it happened to be in the month of Ramaḍān or outside. 
If he does strive his best to determine the month and then fasts, the following rulings apply:
1) He does not come to know whether his fasting fell in the month of Ramaḍān or not and he remains confused. In this case, there is no difference amongst the scholars that his fasts are valid, and he does not need to repeat them.
2) He came to know that his fast fell in the time of Ramaḍān. The majority of scholars believe his fast to be valid.
3) He came to know that he fasted after the month of Ramaḍān already passed. There is a consensus (ijmā’) amongst the scholars that the fast is valid, for he fasted with the intention of Ramaḍān after it became obligatory upon him. The difference of opinion of whether he must make up for the fasts due to necessity does not exist in this case, for the intention of fasting takes its place.
4) That all the fasts are completed before the month of Ramaḍān. According to the majority, the fasts are invalid.
The Mālikī scholar Abdul-Mālik, along with some of the Shāfi’ī scholars held the view that the fasts are valid. They believed this to be the case because the person fulfilled the obligation of striving his best to determine the time of Ramaḍān.
This is the stance I also adopt due to the following proofs:
“Allāh does not burden a soul more than it can bear.” (Al-Baqarah: 286)
This person strove his utmost. Also from the Sunnah, we have the statement of the Prophet (ṣallallāhu `alayhi wa-sallam):
“When I order you to do something, do as much of it as possible.” 
This person did what he could without falling short. It has been authentically reported in Al-Bukhārī and Muslim  on the authority of Adī ibn Hātim that he said: When “…until the white thread of dawn appears to you distinct from the black,” (Al-Baqarah: 187) was revealed, I said: O Messenger of Allāh, I have placed two threads under my pillow, one white and one black, so that I can differentiate between night and day. The Messenger of Allāh (ṣallallāhu `alayhi wa-sallam) said:
“Indeed your pillow is very wide! What is meant here is the blackness of night and the whiteness of day.”
The Prophet (ṣallallāhu `alayhi wa-sallam) did not order him to repeat his incorrect fasts. Adī strove his best to understand the verse and fasted according to his understanding, which the Prophet (ṣallallāhu `alayhi wa-sallam) then explained was incorrect. However, he did not order him to repeat his fasts.
The same can be said about those who did their best to determine the time of Ramaḍān without falling short in this obligation. If they happened to fast before it, I believe their fast to be valid and they need not repeat it.
However, if they repeat the fasts to be on the safe side, this is something good, but if they suffice with the fasts they already performed prior to the month of Ramaḍān, this is sufficient, Allāh willing.
However, we must mention that if one comes to know the arrival of Ramaḍān by some authentic means, he must fast, even if he had already fasted a month before it, for he is addressed in the saying of Allāh:
“…so whosoever witnesses the month must fast.” (Al-Baqarah: 185)
5) If he fasted the night instead of the day, the scholars are in agreement to the fact that he must make up for them.  If it was not for this consensus, I would have said differently.
Those who came to know that it was Ramaḍān after sunrise or at the time of the sun’s zenith.
There is a consensus amongst the scholars that if it is ascertained at night that the month of Ramaḍān has arrived, but people did not come to know of this until daytime, or it was ascertained during the day, those who did not commence the fast need to refrain from those things which break the fast until sunset, and they must make up the day. The same is said for those who came to know of the arrival of the month some days later.
The fast of one who commits fornication during the day of Ramaḍān, one who is forced to break his fast, and an apostate.
There is a consensus amongst the scholars that one who fornicates with a Muslim or non-Muslim [man or] woman during the day of Ramaḍān must refrain from things which break the fast until sunset, must make up the fast, and must offer the required expiation.
If one was coerced into breaking his fast and he does so, he must refrain from those things which break the fast until dusk. He must also make up for the day later according to the majority of scholars. However, there is no sin upon him, for the fast is performed for Allāh alone, [as in the Qudsī ḥadīth]: “Fasting is for Me….” 
If one is unable to fast due to some resulting harm from others and thus breaks his fast, there is no sin upon him, but he must make up for it some other day. The coerced becomes like the ill in their inability and also in the obligation of making it up. 
According to the Shāfi’ī scholars, the fast is not broken, due to the fact that it was done out of coercion, and in these cases, the result of the action is not given any regards. His action was something prescribed, not something prohibited. His case is like one who forgets, rather he is more deserving of the ruling that the fast is not broken, for he has been commanded to eat in order to repel the harm which results from coercion.
This is the stance I take due to the above reasoning. The same is said for those who are coerced into having sexual intercourse while fasting. 
As for becoming an apostate, there is a consensus that the fast becomes invalid.  Based upon this, a person who commits apostasy  during the day of Ramaḍān has invalidated his fast.
Must he make up for the fast after his repentance from the act which made him an apostate?
There is a consensus that a disbeliever who embraces Islam is not required to make up for missed acts of worship, and there is no difference of opinion that an apostate is a disbeliever. The Shāfi’ī scholars held the opinion that making up the fast is obligatory for the apostate and not a disbeliever.  This is also one of the narrations of Imām Aḥmad, in opposition to the majority. 
However, the following statement of the Prophet (ṣallallāhu `alayhi wa-sallam) counters this opinion: “Islam wipes out the sins committed before it.” 
Abū Bakr (raḍiyallāhu anhu) did not order those who apostated to make up for the acts of worship they missed.
The fast of those who accept Islam after the start of Ramaḍān and during the day.
If a disbeliever accepts Islam in Ramaḍān, there is a consensus amongst the scholars that he is not to make up for the fasts he missed. There is also a consensus that he must fast the remainder of the month. 
However, they differed whether one who accepts Islam during the daytime must refrain from those things which break the fast and make up for it.
Abū Ḥanīfah, Ath-Thawrī, and Al-Awzā’ī held the opinion that the person must refrain for the rest of the day. This is also one of the two narrations of Aḥmad. They reasoned that if the cause for fasting is present before Fajr, the fast would be obligatory, and if the reason came into existence during daytime, it would be obligatory to refrain. An example of this is coming to know the month of Ramaḍān [during the day].
In Al-Ikhtiyār, it states that it is not obligatory upon a disbeliever to fast that day. If a new-Muslim fasts, it is invalid due to the fact that he was not one whose fast would be valid at its beginning, and obligations cannot be broken into parts. But he must refrain for the rest of the day so people do not accuse him, as it is an obligation to stay clear of things which lead to accusations. 
Mālik and Ash-Shāfi’ī held the opinion that it is not an obligation upon him to refrain for the rest of the day, for it was permissible for him to not fast at its beginning, both internally and externally. If he was not fasting, he may continue this state, as in the case of others who do not fast due to valid reasons. This is also one narration of Aḥmad. However, refraining is regarded as better.
Must he make up for that fast some other time?
The Ḥanafī scholars, Mālik and Abū Thawr are of the opinion that it is not an obligation to make up for it. They stated that the person was not able to start the worship at its prescribed time, and thus it is like the one whose excuse ends after the time for the worship has passed.
The Shāfi’ī scholars held two opinions, the correct opinion of the school being that it is not obligatory.
The Ḥanbali scholars held two opinions, and in Manār As-Sabīl, the preferred stance is that one must make it up. 
I hold the view that it is obligatory for the person to refrain due to the sanctity of the month, and so this sign of Islam is now apparent on him, as it is impermissible for a Muslim to involve in anything which breaks the fast during the daylight hours of Ramaḍān without a valid excuse, and this person’s excuse no longer exists.
This is the case even if it is not obligatory upon him to make up for the missed fast, according to the statements of the scholars of Fiqh mentioned previously.
Many companions accepted Islam during various days of Ramaḍān, but there is no report in which the Prophet (ṣallallāhu `alayhi wa-sallam) ordered them to make up for it.
Is work a valid excuse for not fasting?
To begin with, I say that work is not a valid excuse for not fasting, and there is a consensus amongst the scholars in this, as rarely do you see someone not working in Ramaḍān due to fasting, whether that be in Muslim lands or in non-Muslims countries, despite the fact that in some non-Muslim countries they do not believe in our fast nor do they hold any regard for the exhaustion which results from it. They do not hold any regards for our feelings by shortening or changing work hours for those fasting. 
What would be the condition of the fasting person in this situation?
If he is able to fast without incurring illness and he is not in fear of some harm or death, the fast remains an obligation and it is not permissible for him to break his fast at all. Most of the Muslims in non-Muslim countries fall into this category. 
If he is unable to fast, he should take his yearly vacation during Ramaḍān so he can fast. This is the obligation if he is able to do this. If he cannot do any of the above mentioned and leaving his job will not cause any legitimate harm to him, his family or his dependents, then it is obligatory upon him to resign from his work.
If he is not able to do any of this, and he is not able to think of any other way to fast while he is working, such as changing his job or work location, he can continue his work with the intention of fasting every day. If he becomes unable to complete his fast during work, he may break his fast when he becomes almost sure that it will harm him or through experience. He may also heed the advice of a good upright Muslim doctor that fasting during work will result in death, illness or some severe harm, in which case he may make up the fasts during his vacation time and need not leave his work. In Fiqh As-Sunnah, it states:
“It is permissible for those people who have laborious work to break their fast. If fasting exhausts them and is extremely difficult throughout the year, they need not make it up, but rather they may offer expiation for it.”