When precisely did the Pepsi Perfect first appear in Back to the Future: Part II?  I set out to answer that question definitively.

It is 4:29 PM 10/21/15 at the 4:45 mark of the film.

Marty receives his Pepsi Perfect at the 13:40 mark.

That is 8 min 55 sec after we see the time as 4:29 PM in the DeLorean.  There do not appear to be any time-jumping cuts in the action of the film during this period, except for possibly in this cut:

When the Doc exits the skyway and descends over the city area, the camera cuts to a view from the alley below. It is not entirely clear whether he flies above the city for an unshown period before finding the place he chooses to park. However, this seems unlikely, as there is no skyway above the city streets, so that would probably be a moving violation. It is apparent that he did not drive around the city on the ground, however, because in the shot where he is parking the DeLorean, he is still descending from the sky. So if there was any time skipped here at all, it was probably only a few seconds.

Therefor, it seems reasonable to assume that the time at which Marty receives his Pepsi Perfect in the Cafe ’80s is 4:38 PM Hill Valley local time.
According to the railroad map seen in BTTF 3, Hill Valley is located in the Sierra Nevada mountain range in Northern California. All of California is in the Pacific Time Zone.

Both in the year of the film’s release (1989) and in the year 2015, October 21st falls inside of the Daylight Saving Time period. In 2015, due to changes made to US guidelines on DST in 2005, time changes on the first Sunday in November. In 1989, the guidelines dictated that DST end on the last Sunday in October. Of course, the last Sunday in October of 2015 is on October 25th, four days after Marty visits the future. Since the Uniform Time Act of 1966, California and its localities have never deviated from federal guidelines on the observance of Daylight Saving Time, so it is reasonable to assume that the October 21st 2015 depicted in the film was during Daylight Saving Time.

Therefor, I can say with some confidence that Marty received his Pepsi Perfect at 4:38 PM PDT.

If Pepsi is being really clever, this could be when the product will become available for purchase online.