According to Mordekainen's landmark work on the subject, Temporal Meta-Mechanics And You, time coalesces across a wavefront, a sinuous line of 'present' that divides past from future. Each thing, no matter how large or small, represents a point along the wave. Reversing a point along the wave requires that every point capable of interacting with the reversed point also be reversed to the equivalent position. Due to the interconnected nature of the multiverse, this means that an enormous amount of energy must be expended to turn the past into the future, which is why most magic and psionics are only capable of creating leaps to the past measured in seconds. However, jumping into the future, by contrast, merely requires a point along the wave to advance and wait for the others to catch up, which is why time jumps can be made hours into the future by relatively meager psionics.
By related extension, knowing the future is incredibly difficult and filled with uncertainty due to the unknown propagation of the wavefront representing the present. There are innumerable possibly paths for the present to take, and an unequally innumerable (See Mordekainen's Temporal Equation for the enumeration of cross-temporal probabilities, pg. 1492 of the aforementioned text) possible ways of traversing those paths. However, the past, the area behind the present, is already set in stone. By careful usage of the crystalline properties of temporal Archanics, as originally hypothesized by the Great Wyrm Zykandros, one can retrace the thread of the present back to where it becomes the past. Thus, with a sufficient number of artifacts dating back to the time period in question, one can begin to isolate a particular, highly specific, region of time. Of particular interest in this process are bones. While a stone or crystal that formed during a particular time will certainly contain a link back to the deep past, the essence of such an object changes very little over time. Bone, however, once lived, and that radical differentiation in vital essence allows a skilled arcanist to pinpoint the moment in time where the creature transitioned between life and death.
Once a temporal frame has been pinpointed, the key step in duplicating a particular region of time is to find a place where it still exists. This may seem counterintuitive with the earlier assertion that the present is a constant wave, consuming the future and solidifying into the past, but keep in mind that time is a hyperstructure within which all possible realities that exist, will exist, or have existed, are experienced. There are an uncountably infinite number of alternate realities, some very much like ours, and many very much not- the detailed discussion of which is much better and more completely handled by more complete treatises by other authors of good renown. For the purposes of copying a past already experienced by the current dimension onto a sufficiently morphic demiplane, one must only need to know this particular rule of alternate realities: Any moment in time that exists as the past of any reality, is currently being experienced as the present of another reality. While the proof of this law drove three men, two woman, a dragon, and the entire isle of Lys insane, it's imminently useful to our purposes. We need not actually be able to gate to an alternate reality (a feat requiring astounding power), we merely need to create an empty, highly morphic, blank slate of a demiplane in suitably close proximity to the nexal focus of the reality we wish to mimic. From there, the chaos of the newly formed demiplane will imprint on the emerging present of the aligned reality in accordance with the basic principles of planar resonance and nexal harmonics.
Once this has been completed, the process is self-sustaining, with the typical laws of physics as espoused by natural philosophers taking control and guiding the formation of the future. Settle back, rest easy, and take a moment to admire your creation. Don't rush into to study your little chip of duplicated time too quickly- this is a moment that deserves to be treated with a proper reverence. - Exerpt from Demiplanes, a Magician's Desk Reference