Proleptic

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A Proleptic calendar or era is that calendar extrapolated to dates prior to its first adoption. For example, the Battle of Hastings was fought on October 14, 1066, according to the Julian calendar in use at the time. One could also refer to it by the proleptic Gregorian date of October 8, even though the Gregorian calendar was not actually adopted in England until 1752.

Similarly, dates such as AD 306 or 44 BC are examples of proleptic use of the Anno Domini era, which was developed in 525. Related to "prolepsis", anticipation.