Counting days properly
Date arithmetic looks like the easiest thing on this site and is quietly the most ambiguous. A month has no fixed length, two of the twelve differ between years, and almost every question about a date range has two reasonable answers depending on whether both ends count.
The arithmetic is not hard. Agreeing what the question means is.
A month is a calendar month, not thirty days
The first of February to the first of March is one month either way, and it is twenty-nine days in 2024 and twenty-eight in 2025. Anything that treats a month as thirty days will be wrong by one or two days across February and by one day across half the other months.
That is why a total in months never divides neatly into a total in days. Someone born in June 1990 is 36 years and 2 months old on 21 August 2026 — 434 months, and 13,216 days. Dividing the days by the months gives about 30.45, which is the average length of a month and the length of no month at all.
For anything contractual, the safe move is to say which convention is meant. "Thirty days" and "one month" are different periods and both are used.
Adding a month to 31 January
There is no 31st of February, so something has to give, and there are two defensible answers. Roll the overflow forward and you land on 3 March in a normal year. Add thirty days instead and you land on 2 March.
A third convention, used in some financial contexts, clamps to the last day of the month and gives 28 February. All three are in use, and none of them is more correct than the others — they are answers to slightly different questions.
The practical consequence is that "a month from now" is not a well-defined date at the end of a long month, and a deadline written that way is a deadline someone will argue about. Adding 3 months and 10 days to 21 August 2026 gives 1 December — clean, because August has no overflow problem.
31 Jan + 1 month:
roll forward -> 3 March
add 30 days -> 2 March
clamp to month -> 28 February
Work it out: Date Calculator →
Does the last day count?
Ask how many days between Monday and Friday and you will get 4 or 5 depending on who you ask, and both are right. The difference between them is whether you are counting the gaps or the days.
Elapsed days — the gap — is the difference between two dates, and it is what you want for interest, age and deadlines. Inclusive days — counting both ends — is what people mean when they ask how long something takes, which is why working-day counts here include both dates.
The habit worth forming is stating it: "five working days including today" removes the entire argument. It is also worth knowing that both dates get compared at midnight in any decent implementation, so a time of day never quietly costs you a day.
Work it out: Days Between Dates Calculator →
Working days need a holiday list, and it cannot be published
Weekends are easy: strip out Saturday and Sunday and you have working days. Public holidays are not, because they differ by country, by region within a country, and by year — some move with Easter, some shift when they fall on a weekend, and some are announced.
That is why every honest working-day calculator asks you for a number rather than knowing it. A built-in holiday list is a page that expires, and it expires silently: it keeps giving answers, and they quietly become wrong.
The other assumption worth surfacing is the weekend itself. Saturday and Sunday is not universal, and anywhere with a different working week needs a different calculation entirely rather than an adjustment.
Work it out: Working Days Calculator →
Questions
How many days are in a month?
28 to 31, and February changes between years. A month is a calendar month rather than a fixed number of days, which is why month totals never divide neatly into day totals.
What is a month after 31 January?
It depends on the convention. Rolling the overflow forward gives 3 March; adding 30 days gives 2 March; clamping to the end of the month gives 28 February. All three are used.
Do I count both the start and end date?
For elapsed time — interest, age, deadlines — no, you count the gap. For how long something takes, yes, both ends count. Say which you mean and the ambiguity disappears.
Why do working-day calculators ask me for holidays?
Because public holidays differ by country, region and year, and some move with Easter. A built-in list would go out of date silently while still producing confident answers.
Are leap years handled automatically?
In any correct date calculation, yes — the extra day is simply part of the gap between the two dates rather than something added afterwards.
How is age calculated?
A full year passes on the anniversary of the birth date, which is how everyone counts it. That makes 29 February birthdays a matter of convention rather than arithmetic.