The oldest dating systems were in regnal years, and considered the date as an ordinal, not a cardinal number. For example, a monarch could have a first year of rule, a second year of rule, a third, and so on, but a zero year of rule would be nonsense. Applying this ancient epoch system to modern calculations of time, which include zero, is what led to the debate over when the third millennium began.

As a side-note to the debate on timing of the "turning of the millennium", the arbitrariness of the era itself can be raised. Mathematically, the choice of the zero point on any timeline is an arbitrary one. (However, once chosen, it must be abided.)

In the TV series, The X-Files, there is an episode titled "Millennium", continuing the TV series of the same name. In this episode Dr. Scully mentions that the new Millennium doesn't start until January 1, 2001. She is made fun of, but not suggested to be incorrect. Mulder responds, "No one likes a math geek, Scully."