Cases, cases

In a very complete article about Выражения времени на русском языке I see that to indicate "DURATION ( Imperfective Progressive: Accusative Case).
a) Verbal Duration (duration of the activity of the verb): use the accusative case alone
We worked there a whole week/five hours/a long time
Мы работали там целую неделю/ ...
b) Postverbal Duration: She went to the country for a month. на + accusative
c) Preverbal Duration: I'll call you in a week. через + В.п.
It is rather clear, but I find in other place an exercise : "Целыми днями Вы сидите за вашим английским учебником". And again: "Тем временем опять накопилимь кое-какие письма". In both phrases with the meaning of a), I think, is used Т.п. instead of В.п. I just want to know if I should add that in a) also Т.п. can be used. Thanks.

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Originally Posted by radomir

Целыми днями Вы сидите за вашим английским учебником". And again: "Тем временем опять накопилимь кое-какие письма".

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"тем временем" in the second one is translated as 'meanwhile' and I wouldn't try to fit it in some category.

For the first, I feel that instrumental is used when exact duration is unknown in case of repetitive action. 'днями', 'ночами', 'неделями' is ok, 'понедельниками' is not.

Thank you.

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Originally Posted by vox05

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Originally Posted by radomir

Целыми днями Вы сидите за вашим английским учебником". And again: "Тем временем опять накопилимь кое-какие письма".

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"тем временем" in the second one is translated as 'meanwhile' and I wouldn't try to fit it in some category.

For the first, I feel that instrumental is used when exact duration is unknown in case of repetitive action. 'днями', 'ночами', 'неделями' is ok, 'понедельниками' is not.

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Not necesserily unknown.
We say:
утром, днём, вечером, ночью;
весной, летом, осенью, зимой.
Parts of day and seasons require instrumental to express "when an action takes place".

Note that
этим временем, тем временем
are not the same as в это время, в то время.
The latter simply mean "at this time, at that time" while the former are more like "meanwhile", correct.

BTW, I have a question to native English speakers.
There is a common word for "spring-summer-autumn-winter": season ("время года" in Russian).
In Russian we also have a common word for "morning-day-evening-night" which is "время суток". How to say it in English?

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Originally Posted by Боб Уайтман

In Russian we also have a common word for "morning-day-evening-night" which is "время суток". How to say it in English?

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The only expressions I know of for this are "time" or "time of day" or "any time".
ex.
--What time of day does your plane arrive?
--I really don't remember. It could be any time.

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Originally Posted by Боб Уайтман

In Russian we also have a common word for "morning-day-evening-night" which is "время суток". How to say it in English?

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You could say "part of the day."
What part of the day was it?
It was in the morning.

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Originally Posted by sperk

You could say "part of the day."
What part of the day was it?
It was in the morning.

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Thank you, Sperk!
That means I was close enough :)