Yes.
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No, it is more correct to say, "What day is it today?"
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It is grammatically correct to say , "I am in school today." This is because you are in the building, not at the building.
Yes, "starting from today" is correct. It indicates that something is beginning on the current day.
Yes, that sentence is grammatically correct.
It is correct, but it would be nicer to use the baby's name, as in "How is Johnny today?"
It depends on the sentence: You will be coming to the lab in the afternoon! Are you coming to the lab in the afternoon? This afternoon, you will be coming the the lab, won't you?
"We were informed it will be corrected today" is a correct sentence. However, it might sound a little better to say "We were informed that it will be corrected today."
Yes, by the end of today is correct. It's not a complete sentence, though.
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
Justin Bieber is coming to waterford today to watch the movie Never say never.