In spoken conversation, they typically refer to times relative to the actual present. When I say "typically" in this sense, I mean that I've
never heard them used otherwise when spoken aloud; others may certainly have different experience in this regard.
Here are two rough, made-up examples that I hope illustrate what I'm talking about:
Bob had to act fast. Now was his only chance to eat the last donut.
"Now" doesn't refer to the
actual (i.e., the reader's) present but refers instead to the relative present of the narrative tense.
Yesterday he'd been too slow, and the donuts were gone before he even made it to the office.
"Yesterday" refers to "the day before the narrative present" rather than the day before the reader's present.
Of course, I'm aware that there's no reason to assume that a story takes place in the reader's present time. The glitch for me arises when the words are used to indicate time relative to the narrative present, rather than in their conversational "time relative to
now" usage. I'm just not used to thinking of them as pure descriptors, and in my head they refer to specific times-relative-to-now by default.
I confess that the McCarthy quote is a paraphrase from memory, but it definitely occurs in the scene I described. I'm also pretty sure that the usage occurs elsewhere in that book, and maybe in
The Outer Dark as well. McCarthy uses certain affectations in his writing, though, so a deliberately quirky word-use would be right up his alley.