Date Calculator (Add / Subtract Days)

The calculator steps forward — or back — by the given number of calendar days from the base date, then reports the resulting date and its day of the week. Adding 30 days to January 1, 2026 lands on January 31, 2026; month and year rollovers happen on their own. The weekday is not decoration. A term that expires on a Sunday is a different practical problem from one that expires on a Wednesday, and you generally want to know that before you commit to a date rather than after.

These are two different operations that people use interchangeably, and they disagree. Adding days is unambiguous: 30 days after January 31 is March 2 in a common year. Adding months is not, because the target month may not contain the day you started on. What is one month after January 31? February 31 does not exist, so the answer has to be either February 28 (clip to the end of the month) or March 3 (overflow into the next). Different systems, contracts and spreadsheet functions choose differently, and they will not agree with each other. The same gap appears at scale. 90 days is not three months: January 1 plus 90 days is April 1 in a common year and March 31 in a leap year, while "three months" after January 1 is April 1 in both. If your agreement says days, count days; if it says months, count months. Do not convert one into the other and hope.

Everything here runs on calendar days: weekends and public holidays are included, because they are still days. Many legal and commercial periods run only on working days, and for those the business-days calculator is the right tool. There is a third convention worth knowing. Several procedural rule sets count calendar days but then provide that when the last day falls on a Saturday, Sunday or legal holiday, the period runs to the next day that is none of those. Under that convention a 30-day period ending on a Sunday actually ends on the Monday. Contracts sometimes copy the same rule and sometimes do not, so check the document rather than assuming a default. Bank and payment deadlines behave similarly. A due date on a Saturday usually functions as the following business day, because the system that processes the payment is closed.

Every term of days has a hidden decision at the front. Does a "30-day notice" served on the 1st expire on the 30th or the 31st? The common convention is to exclude the day the period starts and include the day it ends — you begin counting from the following day. That is what this calculator does when you add: adding 30 days to the 1st gives the 31st, treating the 1st as day zero rather than day one. If your contract instead counts the starting day as day one, subtract one from the number you enter. It is a single day, and it is the single day that most disputes about a deadline turn out to be about.

Any span crossing February 29 is one calendar day longer in real time than the same nominal span in a common year, which is why the answer to "90 days from January 1" changes between years. The rule behind it: a year is a leap year if divisible by 4, except years divisible by 100, unless also divisible by 400. So 2024 was one, 1900 was not, 2000 was, and 2100 will not be. That last case is the one to remember, because a great deal of software was written by people who stopped learning the rule after the first clause. For short terms the effect is invisible. For a 365-day warranty, a long payment schedule, or a date reconstructed backwards from an anniversary years ago, it is exactly one day — and one day is often the whole question.