How does a prisoner pray in a dark underground cell, knowing nothing about prayer times or when the month of Ramadan begins?

For someone who is uncertain about prayer times or fasting, such as a prisoner, it is obligatory to make an effort to determine them. It is permissible for him to follow the opinion of a trustworthy person who reports based on direct observation, not on mere conjecture, provided that the person himself is unable to make an independent judgment. If he prays or fasts without making such an effort, he must repeat it. Regarding fasting, he must exert effort to determine the beginning of the month and should not follow another who is merely exercising personal judgment like himself. If he fasts based on his own judgment, his fast is sufficient in most cases, whether it coincides with Ramadan (definition) or comes after it. However, if it coincides with a time before Ramadan, there is a difference of opinion regarding its sufficiency, with the more preponderant view being that it is not sufficient, and a make-up fast is obligatory. If he fasts voluntarily or as a vow, and it then coincides with the Ramadan fast, it does not count for the obligatory fast according to the majority of scholars, whereas it does count according to the Hanafis.