On the twenty-eighth day of the fourth month, 1253, Nichiren Daishonin established the teaching of Nam-myoho-renge-kyo at Seichō-ji temple in his native province of Awa, and later he returned to Kamakura, the seat of the military government, to begin propagation. In examining the records, we find that in those days the era names were changed frequently. The year 1253 was in the Kenchō era. Three years later, in 1256, the era name changed to Kōgen, and the next year, to Shōka. Then, two years later, in 1259, it was changed to Shōgen, the following year to Bunnō, and the year after that to Kōchō. In the five years from 1256 to 1261, the era name changed five times. An era name was usually changed only on the accession of a new emperor, or when some natural disaster of severe proportions occurred; the frequency of these changes attests to the magnitude of the disasters that struck Japan during this period.
Link to the relevant part of the Gosho.
Part 5 (of 5): This episode.