Original sentence: Other notable proponents include Hans-Ulrich Niemitz, Christoph Marx, Angelika Müller, and Manfred Zeller.The theory also stems from a claim of Illig's to the effect that by the time the Gregorian calendar was introduced in AD 1582, the old Julian calendar, introduced by Julius Caesar, should have produced a discrepancy of thirteen days between it and the real (or sidereal) calendar, while the astronomers and mathematicians working for Pope Gregory found that the civil calendar needed to be adjusted by only ten days.The validity of the European chronology A dendrochronological attempt to verify the current chronology with published tree ring data
Edited sentence: The theory also stems from a claim of Illig's to the effect that the by the time the Gregorian calendar was introduced in AD 1582, the old Julian calendar, introduced by Julius Caesar, should have produced a discrepancy of thirteen days between it and the real (or sidereal) calendar, while the astronomers and mathematicians working for Pope Gregory found that the civil calendar needed to be adjusted by only ten days