AD 1 in the context of "First-century"

Ab urbe condita (Latin: [ab ˈʊrbɛ ˈkɔndɪtaː]; 'from the founding of the City'), or anno urbis conditae (Latin: [ˈannoː ˈʊrbɪs ˈkɔndɪtae̯]; 'in the year since the city's founding'), abbreviated as AUC or AVC, expresses a date in years since 753 BC, the traditional founding of Rome. It is an expression used in antiquity and by classical historians to refer to a given year in Ancient Rome. In reference to the traditional year of the foundation of Rome, the year 1 BC would be written AUC 753, whereas AD 1 would be AUC 754. The foundation of the Roman Empire in 27 BC would be AUC 727. The current year AD 2025 would be AUC 2778.

Usage of the term was more common during the Renaissance, when editors sometimes added AUC to Roman manuscripts they published, giving the false impression that the convention was commonly used in antiquity. In reality, the dominant method of identifying years in Roman times was to name the two consuls who held office that year. In late antiquity, regnal years were also in use, as in Roman Egypt during the Diocletian era after AD 293, and in the Byzantine Empire from AD 537, following a decree by Justinian.

The terms anno Domini (AD) and before Christ (BC) are used when designating years in the Gregorian and Julian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord" but is often presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ". The form "BC" is specific to English, and equivalent abbreviations are used in other languages: the Latin form, rarely used in English, is ante Christum natum (ACN) or ante Christum (AC).

This calendar era takes as its epoch the traditionally reckoned year of the conception or birth of Jesus. Years AD are counted forward since that epoch and years BC are counted backward from the epoch. There is no year zero in this scheme; thus the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525 by the Eastern Roman monk Dionysius Exiguus but was not widely used until the 9th century. Modern scholars believe that the actual date of birth of Jesus was about 5 BC.

The first millennium of the anno Domini or Common Era was a millennium spanning the years 1 to 1000 (1st to 10th centuries; in astronomy: JD 1721425.5 – 2086667.5). The world population rose more slowly than during the preceding millennium, from about 200 million in the year 1 to about 300 million in the year 1000.

In Western Eurasia (Europe and Near East), the first millennium was a time of great transition from Classical Antiquity to the Middle Ages. The 1st century saw the peak of the Roman Empire, followed by its gradual decline during the period of Late Antiquity, the rise of Christianity and the Great Migrations. The second half of the millennium is characterized as the Early Middle Ages in Europe, and marked by the Viking expansion in the west, and the continuation of the Byzantine Empire (Eastern Roman Empire) in the east.

A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar (or in its predecessor, the Julian calendar); in this system, the year 1 BC is followed directly by year AD 1 (which is the year of the epoch of the era). However, there is a year zero in both the astronomical year numbering system (where it coincides with the Julian year 1 BC), and the ISO 8601:2004 system, a data interchange standard for certain time and calendar information (where year zero coincides with the Gregorian year 1 BC; see: Holocene calendar § Conversion). There is also a year zero in most Buddhist and Hindu calendars.