Abstract: A time period calculator employing a base, a calendar disc, a calculation disc and a cursor. The calendar disc includes divisions identified by months and days of the year about a common axis. The calculation disc is divided into increments of time in days, weeks and months from zero consecutively in both clockwise and counterclockwise directions for measuring time periods prospectively and retrospectively, respectively. The base may include divisions divided into 1/365th segments with day of the week indications thereon. The cursor includes line segments offset over the calendar disc divisions by one day so as to accommodate leap year calculations.