How this calculator works
One hundred days is a common planning horizon: the first 100 days of a job, government or project, and a round length for a challenge. The calculator adds or subtracts exactly one hundred calendar days, so the answer does not depend on month lengths, and the date table repeats the same step from the first of each month so you can compare start dates at a glance. The weekdays-inside figure counts Monday to Friday dates after the start through the result and does not remove holidays.
Formula
result = fromOrdinal(O(start date) + 100) forward, or fromOrdinal(O(start date) - 100) backward. The span is 14 weeks and 2 days. Weekdays inside counts Monday to Friday dates in the half-open interval between the two dates.
Worked example
100 days after 2026-11-01 is 2027-02-09, a Tuesday, 14 weeks and 2 days later with 72 weekdays inside. 100 days before 2026-11-01 is 2026-07-24, a Friday. Counting the start date as day one moves the forward answer to 2027-02-08.
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Frequently asked questions
What is the date 100 days from today?
Set the start date to today and leave the direction on forward. The result is the calendar date one hundred days later. Because the page cannot know today from a static file, it starts from the date shown and you change it as needed.
What is a first-100-days plan?
A first-100-days plan sets early priorities, listening sessions and quick wins for a new leader or team. The 100-day date marks the review point, so this page gives both the date and the working days available before it.
Is 100 days about three months?
It is a little more. Three calendar months run from 89 to 92 days depending on the months, while 100 days is roughly 3.3 average months. Use the 6 month calculator or the month counter when months are what the agreement specifies.
Does the 100th day include the start date?
By default the start date is day zero, so the 100th day is 100 calendar days later. The include option makes the start date day one, which is the convention some challenge plans use.