What civilization was the first to create a form of a calendar and how is it similar or different to the system that we use today?

The first civilization that used a calendar were the ancient Egypt.

The Egyptians organized their calendar from a year divided into three different seasons, which were formulated from the variation of the waters of the Nile. They basically worked with the flood, sowing and harvest seasons. As they needed to anticipate the occurrence of each of these times, the constant observation of the stars also served as a reference. Already in the 5th century BC, the Egyptians adopted a calendar with 365 days and subdivided into 12 months with 30 days.