Illig was born in 1947 in Vohenstrauß, Bavaria.
He was active in an association dedicated to Immanuel Velikovsky, catastrophism and historical revisionism,
Gesellschaft zur Rekonstruktion der Menschheits- und Naturgeschichte (Eng: Society for the Reconstruction of Human and Natural History).
From 1989 to 1994 he acted as or of the journal Vorzeit-Frühzeit-Gegenwart.
Since 1995, he has worked as a publisher and author under his own publishing company, Mantis-Verlag, and publishing his own journal, Zeitensprünge (Eng: Leaps in Time).
Outside of his publications related to revised chronology, he has ed the works of Egon Friedell.
Before focusing on the early medieval period, Illig published various proposals for revised chronologies of prehistory and of Ancient Egypt.
His proposals received prominent coverage in German popular media in the 1990s. His 1996 Das erfundene Mittelalter (Eng: The Invented Middle Ages) also received scholarly recensions, but was universally rejected as fundamentally flawed by historians.
In 1997, the journal Ethik und Sozialwissenschaften (Eng: Ethics and Social Sciences) offered a platform for critical discussion to Illig's proposal, with a number of historians commenting on its various aspects.
After 1997, there has been little scholarly reception of Illig's ideas, although they continued to be discussed as pseudohistory in German popular media.
Illig continued to publish on the "phantom time hypothesis" until at least 2013.
Also in 2013, he published on an unrelated topic of art history, on German Renaissance master Anton Pilgram, but again proposing revisions to conventional chronology, and arguing for the abolition of the art historical category of Mannerism.
The scarcity of archaeological evidence that can be reliably dated to the period AD 614–911, the perceived inadequacies of radiometric and dendrochronological methods of dating this period, and the over-reliance of medieval historians on written sources.
The presence of Romanesque architecture in tenth-century Western Europe, suggesting the Roman era was not as long ago as conventionally thought.
The relation between the Julian calendar, Gregorian calendar and the underlying astronomical solar or tropical year. The Julian calendar, introduced by Julius Caesar, was long known to introduce a discrepancy from the tropical year of around one day for each century that the calendar was in use. By the time the Gregorian calendar was introduced in AD 1582, Illig alleges that the old Julian calendar should have produced a discrepancy of thirteen days between it and the real (or tropical) calendar. Instead, the astronomers and mathematicians working for Pope Gregory XIII had found that the civil calendar needed to be adjusted by only ten days. (The Julian calendar day Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582). From this, Illig concludes that the AD era had counted roughly three centuries which never existed.
The most difficult challenge to the theory is through observations in ancient astronomy, especially those of solar eclipses cited by European sources prior to 600 AD (when phantom time would have distorted the chronology). Besides several others that are perhaps too vague to disprove the phantom time hypothesis, two in particular are dated with enough precision to disprove the hypothesis with a high degree of certainty. One is reported by Pliny the Elder in 59 AD and one by Photius in 418 AD. Both of these dates and times have confirmed eclipses. In addition, observations during the Tang dynasty in China, and Halley's Comet, for example, are consistent with current astronomy with no "phantom time" added.
Archaeological remains and dating methods such as dendrochronology refute, rather than support, "phantom time".
The Gregorian reform was never purported to bring the calendar in line with the Julian calendar as it had existed at the time of its institution in 45 BC, but as it had existed in 325, the time of the Council of Nicaea, which had established a method for determining the date of Easter Sunday by fixing the vernal equinox on March 21 in the Julian calendar. By 1582, the astronomical equinox was occurring on March 10 in the Julian calendar, but Easter was still being calculated from a nominal equinox on March 21. In 45 BC the astronomical vernal equinox took place around March 23. Illig's "three missing centuries" thus correspond to the 369 years between the institution of the Julian calendar in 45 BC, and the fixing of the Easter Date at the Council of Nicaea in AD 325.
^Fomenko, Anatoly (2007). History: Chronology 1: Second Edition. Mithec. ISBN2-913621-07-4.
^Johannes Fried: Wissenschaft und Phantasie. Das Beispiel der Geschichte, in: Historische Zeitschrift Band 263,2/1996, 291–316.
Matthias Grässlin, Frankfurter Allgemeine Zeitung 1. Oktober 1996
^EuS 1997 Heft 4.
Theo Kölzer (Bonn University) refused to contribute, and the journal printed his letter of refusal instead in which Kölzer criticizes the journal for lending credibility to Illig's "abstruse" idea.
A favourable review was published by sociologist Gunnar Heinsohn, which later led to a collaboration between Illig and Heinsohn until 2011, when Heinsohn left the board of ors of Illig's journal and published his rejection of Illig's core idea that the figure of Charlemagne is a high medieval fiction.