Add Days Calculator

What a batch date adder does that a single date adder cannot

Most date arithmetic is one date and one offset. Real jobs are often a list instead: every lease start date plus 60 days, every invoice date plus 30, every school term date plus 90. Doing those one at a time is where mistakes creep in, because the same offset lands on a different weekday each time.

This calculator applies one offset to a whole list of start dates. Each date gets its own row with the result, the weekday it lands on, the ISO week number, the calendar days in the span, the workdays, the weekend days and any federal holidays skipped.

The other date pages answer narrower questions, so pick the right one first.

Batch mode also removes a class of copy and paste errors. When the same 90 day rule applies to 40 records, one list means one answer key, and the weekday shift is identical on each row.

How to add days by hand without borrowing the wrong month

Adding days is easy when you keep the year and month and let the day number carry forward. Add N to the day number, then while the number is larger than the length of the month you are in, subtract that month's real length and step into the next month. Step past December and the year goes up by one. The part people get wrong is the month length they subtract.

November 3, 2026 plus 90 days: November has 30 days, so 3 plus 90 gives 93, which is larger than 30, so take 30 off and move to December with 63 left. December has 31 days, so 63 minus 31 leaves 32 and the month becomes January. January also has 31 days, so 32 minus 31 leaves 1. The answer is February 1, 2027.

Counting back works the same way with the sign flipped, and that is where the borrowing rule matters. Subtract the year, the month and the day separately. If the day comes out negative, borrow the length of the month immediately before the end month, the month you are stepping back into. If the month is then negative, borrow 12 months and drop a year.

March 2, 2027 minus December 28, 2026 gives days of 2 minus 28, which is negative 26. Borrow the length of February 2027, the month before the end month of March, and February 2027 has 28 days, so negative 26 plus 28 leaves 2 days. The months are 3 minus 12, which is negative 9, and the day borrow makes it negative 10, so borrow 12 months to get 2. The years are 1 minus 1, which is 0. The result is 2 months and 2 days, and December 28 plus 2 months is February 28, plus 2 days is March 2, 2027.

A flat 30 day month is the most common error in hand written date subtraction, and it is why this page shows the month it borrowed from. In a leap year that month is one day longer, so March 2, 2028 minus December 28, 2027 is 2 months and 3 days, because February 2028 has 29 days.

A batch of five dates with 90 days added to each

The calculator running with five start dates and an offset of 90 calendar days. Each row is worked out on its own, so the weekday differs from row to row even though the offset is identical.

Two things stand out. The result weekday changes but the shift does not: 90 days is 12 weeks and 6 days, so every result sits 6 weekdays later than its start, which is why Tuesday becomes Monday. The workdays column moves only between 64 and 65, because where those 6 extra days fall decides the split.

The same 90 day offset is 2,160 hours, or 129,600 minutes, and that holds only if no daylight saving change falls inside the window.

Business day mode: what changes and what does not

Business day mode changes the length of the journey, not the direction. Weekends are skipped, and with the holiday switch on, federal holidays are skipped too. A 90 business day offset is 18 working weeks, so a Monday start with no holidays inside lands 126 calendar days later.

Read the difference between the last two columns. From November 3, 2026, 90 business days with weekends only lands on March 9, 2027, and the same count with federal holidays skipped lands on March 17, 2027, because six holidays sit inside that span: Veterans Day, Thanksgiving, Christmas, New Year's Day, Martin Luther King Jr Day and Washington's Birthday.

State holidays, court closures and company shutdowns are not on the federal list, so the holiday switch can miss a real deadline. If your rule depends on a state calendar, treat this as a close reading.

The federal holidays the counter skips

The holiday list is built from the rule rather than a stored table, so it stays right in any year you type. Federal law in 5 U.S.C. 6103 names the eleven public holidays, and most are defined by position, such as the third Monday in January. The table shows the observed dates for 2027 as listed by the Office of Personnel Management.

The Saturday rule comes straight from the statute, and the federal holiday schedule published by the U.S. Office of Personnel Management explains it in plain words: a holiday on a Saturday gives most federal employees the Friday before, a holiday on a Sunday gives them the Monday after. The law itself is worth reading at 5 U.S.C. 6103 on the US Code site. The December 31, 2027 row surprises people: it sits in the 2027 schedule because the day off lands in 2027.

Common deadline lengths and how far the weekday moves

The offsets that show up most often in contracts, policies and court rules are worth checking once. The table starts from November 3, 2026 and shows what each one works out to, including how many weekday positions the result moves, which is the remainder when you divide by 7.

Offsets that are a multiple of 7 land on the same weekday, so a 28 day or 56 day notice period never changes the day of the week. The others drift: 90 days lands 6 weekdays later, and if the rule also rolls a deadline to the next working day, the adjustments stack.

365 days lands on the same date a year later but on a different weekday, because 365 is 52 weeks and 1 day. The 366 days a leap year adds to a same-date window move the weekday by 2, and 364 days returns to the same weekday.

Reading the ISO week number in the results

ISO week numbers run Monday to Sunday, and week 1 holds the first Thursday of the year. That rule is why a January date can belong to the previous year. November 3, 2026 plus 60 days lands on January 2, 2027, which sits in week 53 of 2026. The next Monday, January 4, 2027, starts week 1 of 2027.

The same happens at the other end of the year. December 31, 2026 falls in week 53 of 2026, while December 28, 2026 plus 7 days lands on January 4, 2027 in week 1 of 2027. Some systems label weeks differently, so check the convention first.

The week number labels a stretch of time in a way a date cannot. The NIST Time and Frequency Division keeps the reference time standards behind the calendar.

Making the count stick when a deadline depends on it

Four details decide whether you land on the right day, and all four come from the rule you are working from. When does the clock start? Some rules count from the event, others from the day you got notice. Is the last day included? Most filing windows include it. What happens when the last day is a weekend? Some rules roll forward, some do not. Which clock matters? A mailed filing goes by the postmark, an online filing by the server time.

For a batch of deadlines, write the rule down once, run every start date through the same setting, and save the result. Each row shows the weekday, so a weekend landing is visible.

One last warning about the calendar days column. It counts the days between the two dates, which is not the number you asked for once weekends are skipped. A 90 business day request can cover 129 to 134 calendar days, and that gap is why the two modes exist.

This page is a date arithmetic tool, not legal advice. The deadline printed in the document you are working from always controls.