The feat Keen Mind (PHB, page 167) says:
You have a mind that can track time, direction and details with amazing accuracy. You get the following benefits.
- Increase your Intelligence score by 1, to a maximum of 20.
- You always know in which direction the north is.
- You always know how many hours are left before the next sunrise or sunset.
- You can remember with precision anything that you have seen or heard in the last month.
This seems to imply that, by default, a character does do not necessarily know "the amount of hours that are missing before the next sunrise or sunset" or remember what you have read in the books regarding the reading time of the stars, the planets or the position of the sun.
Is there any temporary information that the characters necessarily know by RAW, or does everything depend on the DM's criterion regarding the situation (for example, a knowledge check on astronomy or access to a city square where a calendar can be published)? For example, if PCs have been imprisoned in a windowless cell, they may not know the exact time of the watch or even the correct time of day, but intuitively Know the exact number of calendar days that have elapsed since they were released.