seigokeisan.com

Seigo Keisan.com is a Japanese online date-calculation service that helps users find how many days have passed since a birthday. Enter a date of birth and a target number of days to calculate milestones, from a baby’s 100th or 200th day to a person’s 7,777th or 10,000th day. The site is designed for families tracking a child’s growth and for anyone planning a thoughtful birthday-based surprise for a friend, partner, or relative.

Alongside its calculator, Seigo Keisan.com explains the meaning of common milestones, including the traditional 100-day celebration known as Okuizome and the increasingly observed half-birthday at around six months. It also clarifies that counting conventions differ: medical age calculations treat the birth date as day zero, while traditional ceremonies often count it as day one. Users can choose whether the first day is counted as zero or one, helping them calculate dates according to their needs. The site provides sample calculations for several milestone totals, as well as app-installation information, a privacy policy, disclaimers, and contact details.