Until the 20th century, with its multiple developments, only relative dates could be determined with any confidence. Since the turn of the century, several methods to measure elapsed time have been discovered. The first and simplest method of absolute dating is using objects with dates inscribed on them, such as coins, or objects associated with historical events or documents. Most chronological markers are based some cyclical event. Many of these are astronomical events.
Calendars are usually simple methods for dividing up years.
Often based on moon phases, solstices. Astronomical observatories were fairly common. Most of our reckoning of dates follows some system like them. Some chronological markers provide a link to the calendar. For example, since each Roman emperor had his own face stamped on coins during his realm, and dates for emporer's realms are known from historical records, the date a coin was minted may be discerned by identifying the emperor depicted .