Quote Originally Posted by wonlon View Post
I checked some examples, some are singuar, some are plural, but seems to be of the same type.

по году не писал писем
сидеть над книгой по целым дням
Не видал его по целым неделям

In my Russian-Chinese dictionary it says on по + dat. in this meaning: "during the whole period, meaning the duration/length of time the action lasts".

I wonder if по утру means something like "the whole morning", thus the above sentence "you cannot be without the daybreak for the whole morning."

But утром does not always mean something that lasts the whole morning, like утром я чищу зубы.

Am I correct?
What's that in bold? I'm afraid none of that makes much sense.

Годами не писал писем - haven't/hasn't written letters for years.
Сидеть над книгой целыми днями - to sit reading the book for whole days.
Не видел его целую неделю - haven't/hasn't seen him for a week.

As noted, поутру is the same damn thing as утром - in the morning. That's pretty much it.

Note, if you want to say "in the mornings", then you have to say по утрам, you can't say утрами. I guess this is where по in the singular use of the expression comes from.