During the past 1400 years - from 618 to 2018. In 1400 - that's it, only one year, 1400. In 1400 years - in the year 1400 years from now and later on, after 3418 year.

There is not any difference between during the past 1400 Years, In 1400 and 1400 Hundred Years. The whole maters state the past time of the years. All these words of group denotes about the past.

During the past 1400 years: quiere decir en los últimos 1400 años, osea esta en un verbo pasado. In 1400: quiere decir: en 1400 pero no específica que es, puede ser años, cantidad etc. In 1400 years: habla de años quiere decir en 1400 años