The Isawa Calendar measured the passage of years beginning with the founding of Gisei Toshi, an event which defined the year 1. Each successive year since the founding of that city had counted up from year 1. This method was very consistent and had achieved widespread use in record keeping throughout the Empire, even among the Ikoma family historians. The Sparrow Clan used a modified calendar based off of the date of their founding; this was occasionally used by other members of the Three Man Alliance, including the Fox Clan and the Tsuruchi family.