Which part of speech?

I should know this, but my brain keeps coming up empty (like that hasn't
ever happened before.)

In the sentences, "It is raining today" and "Yesterday, she went to the
market", what part of speech is 'today' and 'yesterday'? I want to say
that they are acting as adverbs, but that doesn't seem right.

Thanks in advance.

Gregory Gadow

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