Words like yesterday, today, and tomorrow are defined as adverbs. However, an adverb is a word modifying an adjective or verb (or another adverb). Words such as yesterday do not seem to modify anything of the sorts.
In reading a book about grammar, a sentence is given as an example of the use of adverbs:
"Yesterday the quite relieved soldier very quickly ran out of the woods when he saw his comrade frantically waving to him".
I'll come back to the word 'yesterday'. Apart from that, the book identifies 'quite', 'very', 'quickly' and 'frantically' as adverbs. Let's make a table of the adverb and the thing it modifies:
Adverb Modifies ___________________________________ Quite | Relieved(adjective) Very | Quickly(adverb) Quickly | Ran(verb) Frantically | Waving (verb)
Ok, so back to yesterday (how poetic). Yesterday doesn't actually seem to modify anything. Apparently, it modifies the verb ran, and this is coming from the book. However, we need to note two things here:
Extending the present logic, if it 'modifies' the verb ran, surely it modifies everything else since everything took place yesterday. If it modifies the action of running, then surely it modifies the other action: waving. Perhaps it also modifies other adverbs, and adjectives, too.
What does it mean to modify? Surely the way that an action is executed transcends time. I could run in an identical way to how I ran yesterday or will run tomorrow. Yet the action is supposedly modified depending upon the time it's executed. Well, extending this logic, why not place? And the agent executing the action?
So it doesn't seem correct to say that time modifies adverbs. If anything, it seems more related to pronouns, since a pronoun is a word that stands in for someone else, in the same way 'yesterday' is a word that stands in for everything that happened within the span of 24 hours within the boundaries of our planet earth. So yeah, I don't really see the logic.