[Written some years ago. Not all details may be applicable now. Edited on 12/11/16]
2 Peter 3:8 reads (in part):
AGAPHTOI hOTI MIA hHMERA PARA KURIWi hWS XILIA ETH KAI XILIA ETH hWS hHMERA MIA.
Before I discuss the Witness view of 2 Peter 3:8, I want to point out a number of interesting features associated with this passage.
(1) The hOTI clause evidently signals or introduces an indirect assertion (Cf. John 11:27; Luke 24:21). Peter is contending "that" one day with YHWH (or possibly Jesus) is comparable to a thousand years and a thousand years are comparable to one day.
(2) It is also interesting how Peter begins and ends the indirect discourse in 3:8. He chiastically starts with MIA hHMERA and terminates the clause with hHMERA MIA. The parallelism XILIA ETH (AB) . . . XILIA ETH (AB) also seems to be "nested" by hWS (the two occurrences of hWS serve as "bookends"), though the position of the second hWS is rhetorically "flipped" so that we have, hWS XILIA ETH KAI XILIA ETH hWS, which seems to be an example of antimetabole or literary crisscrossing (My Classics professor used to say that "antimetabole" is "going out the same way that one comes in," rhetorically speaking). The chiasm and rhetorical devices in this verse are fascinating and indicate the writer was somewhat knowledgeable about producing a rhetorical and theological document.
(3) The presence of hWS also shows us that we are dealing with a simile. One thousand years is as a day with God; Peter ostensibly does not mean to assert that a thousand years literally equals a day with God, although I am not denying that this principle has limited applicability in other cases. But in this Scripture, Peter simply appears to be echoing the words of Moses found in Psalm 90:4:
"For a thousand years in your sight are like yesterday when it is past, or like a watch in the night."
Notice that Psalm 90:4 is an example of simile and it declares that a thousand years is both akin to a day ("yesterday") and "a watch in the night." The writer is clearly not inviting us to make direct comparisons between a divine "day" and the creative "days" of Genesis. Most of us can probably agree up to this point. However, let us now move on to what Jehovah's Witnesses have said about 2 Peter 3:8.
"So what is to man a thousand years or a period of more than 365,000 days is, comparatively speaking, like just one 24-hour day to the eternal God" (Choosing the Best Way to Life, page 174).
"What is such a long time to men is really a short time to God. Hence, he can allow to men a seemingly long period of time in their interest. What is a 'thousand years' to Him, when it is like a mere twenty-four hour day in comparison with his eternity of existence?" (Man's Salvation Out of World Distress At Hand, page 298).
The book Life--How did it get here? By evolution or by creation? does suggest that the creative days in Genesis could have been longer than 24 hours, and it is no secret that past JW publications have said that the "days" in Genesis could have been 7000 years in length. But I do not remember reading that the days were 1000 years each. But even the Life book makes the following statements:
"A thousand years are likened to a day. (Psalm 90:4; 2 Peter 3:8, 10) 'Judgment Day' covers many years . . . It would seem reasonable that the 'days' of Genesis could likewise have embraced long periods of time--millenniums" (page 27).