The Interstitial Period, sometimes called the First and Second Wave Interstitial. The First and Second Wave exploratory periods correlate reasonably well with the transition from the old multinational system to the World Republic.
It goes without saying that these dates are approximate, anything regarding the Interstitial must always be regarded as suspect.
The Interstitial Period is one of the two great periods of lost history in the modern age (post 21st century of the old reckoning). The other obviously being the Aftermath of the Fall. Much like the Aftermath, the Interstitial is characterized by civil unrest, wars, and loss of records of all kinds.
In fact, the Interstitial is by far the more traumatic. During the original explosion of the Information Age (beginning in the 20th century, old reckoning), short-sighted data-barons transferred nearly all forms of record-keeping and public documentation into digital formats, with little to no analog back-up. When the system was brought down by data-terrorists at the onset of the Interstitial Period, nothing remained. Reliable records of those times remain elusive even now, over a millennium later.
It should also be noted that our placement of the Interstitial in the Timeline as it relates to events on Somnius is, at best, an educated guess.