Yes, by the end of today is correct. It's not a complete sentence, though.
No, it is more correct to say, "What day is it today?"
Wednesday morning, it is correct to say I have not seen him for one day. Wednesday afternoon, or by working day end, it is correct to say I have not seen him for two days
Yes, "starting from today" is correct. It indicates that something is beginning on the current day.
Yes, it is correct to write "Please reply by today" to indicate that you are requesting a response by the end of the day.
Yes, you can say that; we've got today to get this job done.
No, it is more correct to say, "What day is it today?"
Today is already implied at nights end & no need to add. Just say "I had a great evening".
Wednesday morning, it is correct to say I have not seen him for one day. Wednesday afternoon, or by working day end, it is correct to say I have not seen him for two days
say it will end "badly."
Neither is correct. We say "at the end of August."
It is grammatically correct to say , "I am in school today." This is because you are in the building, not at the building.
Yes.
Yes, "starting from today" is correct. It indicates that something is beginning on the current day.
Yes, the possessive form of the noun week is the correct form for 'the end of the week' (the week's end).
Yes, it is correct to write "Please reply by today" to indicate that you are requesting a response by the end of the day.
Tag questions can end in it. That's correct, isn't it? or That's not correct, is it?
Yes that is correct.