Here's why:
* Adverbs modify verbs, adjectives, or other adverbs.
* "Today" tells us *when* something happened or will happen.
For example:
* "I went to the store today." (modifies the verb "went")
* "It is a beautiful day today." (modifies the adjective "beautiful")
* "I arrived very early today." (modifies the adverb "early")