An era of the time-line. !(p) Most calendar systems have a single epoch dividing the time-line into two eras. However, some calendar systems, have multiple eras, such as one for the reign of each leader. In all cases, the era is conceptually the largest division of the time-line. Each chronology defines the Era's that are known Eras and a {@link Chronology#eras Chronology.eras} to get the valid eras. !(p) For example, the Thai Buddhist calendar system divides time into two eras, before and after a single date. By contrast, the Japanese calendar system has one era for the reign of each Emperor. !(p) Instances of {@code Era} may be compared using the {@code ==} operator.