Your age in total number of days is calculated as: Number of years, x, with 365 days = 365x plus. Number of years, y, with 366 days = 366y plus. Number of days in the remaining partial year. This calculator assumes the start date is day 0 and begins counting one full day after day 0. For example if you input the dates 03/15/2022 and 3/18/2022 ...