120 Day Calculator

Counting 120 days: include the start date or not

The calculator moves 120 days from any start date, forward or backward, and the first choice is whether the start date itself counts. Count it, and a window that starts on Sunday, March 1, 2026 ends on Sunday, June 28, 2026: the span from March 1 through June 28 inclusive holds 120 days. Skip the start date, and the same window ends on Monday, June 29, 2026, one day later.

The landing weekday follows from the arithmetic. June 28 sits 119 days after March 1, and 119 / 7 = 17, so the target lands on the same weekday as the start. With the start date excluded, the target sits 120 days away, so it moves one weekday later, from Sunday to Monday.

Contract language decides the choice. A clause that says a thing must happen within 120 days of a notice date usually counts from the day after the notice, while a window described as the 120 day period beginning on a date includes that date as day one. When the document does not say, compute both dates and plan against the earlier one.

Worked example: a 120 day window across New Year

Start on Friday, November 20, 2026 and count the start date. Day 120 lands on Friday, March 19, 2027, and the window crosses the calendar year on the way. The span holds 2,880 hours, since 120 x 24 = 2,880, and it contains 86 weekdays and 34 weekend days. Skip the start date and the target moves to Saturday, March 20, 2027, with 85 weekdays and 35 weekend days inside the window.

The shape of the span stays constant: 120 days is 17 full weeks plus 1 day, since 17 x 7 + 1 = 120. That leftover day is the only reason two 120 day windows can end on different weekdays or hold a different weekday count. The window from November 20, 2026 passes through December, January, February and March of 2027, and February 2027 has 28 days, so no leap day appears and nothing shifts.

Planning against the weekday matters when the deadline lands on a Saturday, as the excluded count does here. A deadline of March 20, 2027 on a Saturday usually moves to the next business day, which stretches the effective window by two calendar days.

Counting backward: 120 days before a date

Subtraction runs the same machinery in reverse. Counting backward from Thursday, December 31, 2026 with the start date included, day 120 is Thursday, September 3, 2026. The span from September 3 through December 31 inclusive holds 120 days, 86 weekdays and 34 weekend days. With the start date excluded, the target is Wednesday, September 2, 2026.

Backward counts show up whenever a rule looks back in time: a condition that must have appeared within 120 days of a filing, or an average measured over the 120 days before an event. The same include or exclude question applies, and the two answers sit one day apart. September 3, 2026 versus September 2, 2026 is the whole gap, and on a claim the wrong one can put the event inside or outside the window.

One more habit worth keeping: write down which convention you used next to the date you computed. A note reading December 31, 2026 back 120 days, start included, gives September 3, 2026 is checkable months later. A bare September 3 is not.

Leap years and February 29 inside the window

Leap years follow the Gregorian rule: divisible by 4 is a leap year, except century years, unless the century divides by 400, as the U.S. Naval Observatory's calendar introduction explains. 2024 and 2028 are leap years, 2100 will not be. A 120 day window is short, so most contain no February 29 at all; only a window that crosses that date gains a day inside it.

Two checked examples. Start on Saturday, January 1, 2028 and count the start date: day 120 is Saturday, April 29, 2028, and the span includes February 29, 2028, so the calculator flags it. Start on Wednesday, February 28, 2024: day 120 is Wednesday, June 26, 2024, which includes February 29, 2024.

The leap day does not change the day count, which is always 120. It changes how the window sits over the calendar, and it can change a weekday tally, because February 29, 2028 is a Tuesday and adds a weekday to any window containing it.

Weekdays and weekends inside 120 days

The weekday count has a fixed pattern. Because 120 days is 17 full weeks plus 1 day, the 17 weeks contribute 17 x 5 = 85 weekdays and 34 weekend days, and the single extra day is the start weekday itself. A window that starts on any weekday from Monday to Friday therefore holds 86 weekdays and 34 weekend days, while a window that starts on Saturday or Sunday holds 85 and 35.

The examples above confirm the pattern from both directions: March 1, 2026 is a Sunday, so the window holds 85 weekdays; November 20, 2026 is a Friday, so its window holds 86. December 31, 2026 counted backward is a Thursday, so that window holds 86 as well.

Business days are a different unit. One hundred twenty business days is 24 weeks of 5 workdays, which needs about 24 x 7 = 168 calendar days before any holiday. Federal holidays stretch it further, and the OPM federal holiday schedule lists the eleven dates that move deadlines for federal offices. A contract that means business days should say so, because 120 of one is 168 of the other.

120 days against calendar months

People round 120 days to four months, and the two are close but not equal. Using the average Gregorian month of 30.44 days, 120 / 30.44 = 3.94 months. A four month window depends on which months it crosses: four months after March 1, 2026 is July 1, 2026, which is 122 days later, two days past the 120 day mark.

A three month window from the same start runs 30 days short of 120, and a four month window runs two days long. Over a year of quarterly planning those gaps compound. When a deadline is written in days, count days; when it is written in months, count months. The two conventions agree only by accident of which months fall inside.

Mistakes this calculator removes

Four errors account for nearly every wrong 120 day date. First, counting the start date twice: a spreadsheet that adds 120 days to March 1 and then reports June 28 as both day one and the endpoint has double counted one day somewhere. Decide the convention, then let the calculator apply it.

Second, assuming 120 days equals four months. As the table above shows, four months from March 1, 2026 runs 122 days. On a deadline that two day difference decides timeliness.

Third, forgetting the leap day in a 2028 or 2024 window. A window from late 2027 into 2028 that crosses February 29 contains 120 calendar days either way, but hand counting that skips the extra day lands one date short.

Fourth, mixing units: 120 business days is about 168 calendar days, and 120 weekdays is not 120 days. The calculator counts calendar days only and reports the weekdays inside the span, so a rule that means business days needs a different count. When stakes are high, verify the convention in the governing document, compute both include and exclude answers, and keep the earlier date as the safe one.