(How) did the Republican Romans refer to previous decades or centuries?

Since I wrote my thesis about Roman Calendars and Time Reckoning, this is a very pleasant question to answer.

1. Centuries Reckoning

Romans used Saeculi Naturale, Civile and Religiosi. Saeculum means an indeterminate long period, but also a generation.

They used Saeculi Naturale often, to divide society in "generations", not only families as today. This is obviously, a relative dating method.

The Saeculi Civile were time units of Etruscan origin, that were reckoned from the AUC date (foundation of Rome). They were of 100, 110, or 125 years. Yes, they disagree because these were used to calculate the year of celebration of the Ludi Saeculares, a major game event that emperors used to influence Roman Society. So, emperors that wanted to celebrate games, choose the century lenght so they could celebrate the 700, 800th etc year of Rome's birth when they wanted.

The Saeculi Religiosi were the exact interval between Ludi Saeculares already celebrated (they were a sacred event).

2. Years Reckoning

Romans used a variety of systems. They used lustrums (intervals of 5 years), but in a relative dating way, not structured as our decade system. They did NOT use the decada, which is of Greek origin and was used only sparsely, late in the Empire. They did use many systems for year by year counting:

• Ab Urbe Condita (ranging from 759 to 748 BC): years since the mythical foundation of Rome.
• Post reges exactos / post Capitolinam aedem dedicatam (509 BC): years since the monarchy was overthown.
• Eponymous dating by consulates (507 BC - 541 AD): two consul names a year
• By attribution of non-Roman titles (such as Aegyptian ones).
• Pan-Hellenic Festivals (776 BC-IV AD): including but not only the Olympic Games, the Romans used these festivals, the same as the Greeks and other Greek-influenced Mediterranean peoples.
• Eastern Hellenistic City-State Eras
• By appropriation of magistratures (1st century BC- 4th century)

Methods used also later in Roman history:

• The Cycles of Indictio (297 AD to year 1806)
• Regnal Years (from 537 AD-onwards, specially in the east)
• Western Latin Era (in Hispania)
• Religious Eras (many of them, not a widespread use really)

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