Hypernym for words that refer to a specific point in time like now, today, tomorrow, and yesterday

I believe that the term you are after is temporal deictic expressions.

Here the term deictic may be explained as follows:

Consider the words like I, now, and here. These are paradigmatically deictic terms. By this we mean that their denotation changes depending on who is speaking, when, and where. For example, when John says I, then I denotes John; but when Kim says I, then I denotes Kim. You can think of deixis as being about the 'location' from which the speaker 'points' to things he or she is talking about (indeed, the OED's definition for deixis is 'indication, pointing out'). And note that in order to fully understand what the speaker is saying, we must know this 'location'.

So yesterday means different things depending on when the speaker uses that word. The linguistic terminology for expressing the fact that yesterday has this property is to say that yesteday is deictic.

The relevant segment from CGEL (p. 695):

(a) Deictically - in relation to the time of utterance

[5]  i  I saw her yesterday.
       ii  It'll be all over a year from now.

Yesterday refers to the day preceding the one when I utter [i], and a year from now identifies a period a year later than the time I utter [ii] (see Ch. 17, §10, for a fuller account of temporal deixis). The range of temporal deictic expressions is illustrated in:

[6]  now                    yesterday           today                         tomorrow     this morning
       tonight               last night            tomorrow night      last week       next week
       two days ago     in two weeks     in a week's time      these days     in earlier times

The temporal counterparts of spatial here and there are now and then, but while there is readily used both deictically and anaphorically, then is almost always anaphoric. Such expressions as in two weeks, in a week's time, in earlier times can be used anaphorically as well as deictically. The deictic use is seen in She's arriving in two weeks ("in two weeks from now"), the non-deictic in She was due to arrive in two weeks ("in two weeks from some contextually given point of orientation"). This kind of expression can also have a durational rather than locational meaning: She wrote the report in two weeks.