Here's how to think about it:
* Time as a concept: "Today" is a noun that represents the current 24-hour period. It's a chunk of time, similar to how "week" or "year" are chunks of time.
* Specificity: Unlike "every day" or "any day," "today" points to a specific instance in time.
* Substitution: You can replace "today" with a specific date like "January 1st" or "Tuesday."
Examples:
* "Today is a good day." ("Today" acts as the subject of the sentence, referring to the current day.)
* "What are your plans for today?" ("Today" is the object of the preposition "for" and refers to the current day.)
While "today" can also function as an adverb, its role as a noun is very common and crucial to understanding its meaning in a sentence.