Why use this calculator
Almost every deadline dispute comes down to how the days were counted. Contracts talk in business days, notice periods in calendar days, invoices in terms like net 30, and leases in whole calendar months, and those four counts give four different answers for the same pair of dates.
Counting on a wall calendar works until a leap year, a 31-day month or a year boundary quietly costs you a day. This calculator does all four counts at once from a single pair of dates, shows the weekday each date lands on so you can see whether a deadline falls on a weekend, and keeps a live countdown from today so a date you entered last month still tells you how long is left.
How to use it
- Enter the start date as three numbers: the four-digit year, the month as a number from one to twelve, and the day of the month.
- Read them straight off the document, the contract or the invoice rather than converting them first, because the two-digit-year habit is where most errors start.
- Repeat for the end date in the three matching fields.
- If you type a day that does not exist in that month, such as the thirty-first of February, the calculator trims it to the real last day of the month instead of rolling into the next one.
- The order does not matter: put the later date first and the result shows the same gap with the direction marked as backward.
What each result means
- Days between
- The exact count of calendar days separating the two dates, including leap days and regardless of month lengths.
- Weeks
- The same gap expressed in weeks to one decimal place, which is how leave, gestation and project sprints are usually quoted.
- Calendar span
- The gap broken into whole years, whole months and leftover days, the way a lease or a tenancy agreement describes a term.
- Business days
- Weekdays only, Monday through Friday. This is the count contracts mean when they say working days, before public holidays are removed.
- Weekend days
- Saturdays and Sundays inside the range, which is the difference between the calendar-day count and the business-day count.
- Start falls on
- The day of the week the start date lands on, useful for checking whether a period begins on a non-working day.
- End falls on
- The weekday of the end date. A deadline that lands on a Saturday usually moves to the next business day under most contracts.
- Direction
- Whether the end date comes after the start date (forward), before it (backward), or is the same day.
- Relative to today
- How many days sit between today and the end date, so a saved or shared calculation still reads as a live countdown.
The formula
Days between = (end date as UTC milliseconds minus start date as UTC milliseconds) divided by 86,400,000
Weeks = days divided by 7
Whole calendar months = (end year minus start year) times 12, plus (end month minus start month), minus one if the end day of the month is earlier than the start day of the month
Business days = whole weeks times 5, plus each remaining day whose weekday is not Saturday or Sunday
Working in UTC removes daylight saving, which would otherwise make one day of the year 23 hours long and another 25.
How this number is used in practice
Payment terms are the everyday case. Net 30 means thirty calendar days from the invoice date, not thirty business days, and the difference is roughly twelve days of cash flow on every invoice. Construction and legal contracts usually run the other way and specify business days, which is why a four-week program quoted as twenty days on site consumes twenty-eight days of calendar.
Reading the wrong count into a contract is one of the most common and most expensive administrative mistakes a small business makes.
Statutory periods magnify the difference again because they also strip out public holidays, and those differ by country and by state. The United Kingdom publishes bank holidays centrally, while Australia sets them state by state, so a national deadline can genuinely fall on different working days in Sydney and Perth.
This calculator deliberately does not guess a holiday calendar: it gives you the clean Monday-to-Friday count and leaves you to subtract the specific public holidays that apply where the obligation sits.
For personal planning the weekday rows matter more than the totals. Knowing that a milestone lands on a Sunday tells you to move a delivery, and knowing a lease ends on a Friday tells you the handover inspection has to happen that week rather than the next.
The countdown from today is the row people actually return for, because it turns a fixed date into the only number that changes: how long is left.
Worked examples
A full year, 1 January 2026 to 1 January 2027
365 days, 52.1 weeks, 261 business days and 104 weekend days.
A three-month project, 15 March 2026 to 15 June 2026
92 days, 13.1 weeks, a calendar span of 3 months and 0 days, and 66 business days.
Across a leap day, 1 January 2028 to 1 January 2029
366 days rather than 365, because 2028 is a leap year, giving 52.3 weeks.
A 30-day invoice term, 10 September 2026 to 10 October 2026
30 days but only 21 business days, a gap of 9 days that decides whether you are paid this month or next.
When people use it
- Checking whether an invoice under net 30 terms is genuinely overdue
- Turning a project program quoted in working days into a calendar finish date
- Working out a notice period, probation period or lease term in whole months plus days
- Confirming how many days of annual leave a date range actually consumes
- Counting down to a wedding, settlement, exam or visa expiry
Tips
- Business days here are Monday to Friday only. Subtract the public holidays that apply in your state or country before quoting the figure to a client.
- If a deadline lands on a weekend, most contracts and court rules roll it to the next business day. Check the wording rather than assuming.
- Whole calendar months are not 30-day blocks. The 31st of January plus one month is the 28th or 29th of February, not the 3rd of March.
- Write dates in the ISO order of year, month, day when you share them. It removes the ambiguity between the UK and US day-month conventions entirely.
- Leap years are every year divisible by four, except century years that are not divisible by four hundred, so 2100 will not be a leap year.
- Put the later date in the start fields if you want to look backwards. The day count is identical and the direction row tells you which way you counted.
What it assumes, and where it stops being accurate
- Business days are Monday to Friday. No public holiday, shutdown period or religious calendar is deducted.
- The count runs from the day after the start date up to and including the end date, which is the usual contractual convention.
- All arithmetic is done in UTC, so daylight saving shifts cannot add or remove a day from the total.
- Dates are limited to the years 1900 through 2100, which covers every practical contract, lease and life event.
- A day that does not exist in the chosen month is trimmed to that month's last day rather than rolling into the next month.
Terms used on this page
- Business day
- A weekday on which normal commerce happens, conventionally Monday to Friday excluding public holidays.
- Calendar month
- A month as it appears on the calendar, between 28 and 31 days long, rather than a fixed 30-day block.
- Leap year
- A year containing 29 February, added roughly every four years to keep the calendar aligned with the solar year.
- Net 30
- A payment term meaning the invoice is due thirty calendar days after the invoice date.
- ISO 8601
- The international date standard that writes dates as year, then month, then day, removing regional ambiguity.
Questions people ask
- Does the calculator handle leap years correctly?
- Yes. The count works from real timestamps rather than an assumed 365-day year, so any 29 February inside the range is included automatically. A year that spans a leap day returns 366 days. The leap rule is every fourth year, except century years that are not divisible by four hundred.
- Why do the business days not match my contract?
- Almost always because of public holidays. This calculator removes Saturdays and Sundays but cannot know which holiday calendar applies to you, since they differ between countries and between Australian states. Take the business-day figure here and subtract the specific holidays that fall inside the range.
- Is the start date counted as day one?
- No. The count runs from the day after the start date through to the end date inclusive, which is the convention used by payment terms and most contracts. If your agreement counts the first day as well, add one to every figure shown.
- What does the calendar span row mean?
- It expresses the gap the way a lease does: whole years, whole months, then whatever days are left. A month is a calendar month, so it varies between 28 and 31 days. That is why 92 calendar days can be exactly three months while 90 days is not.
- Can I calculate a date in the past?
- Yes. Enter the earlier date in the end fields and the calculator reports the same gap with the direction marked as backward. The day count, the weeks and the business days are all identical, since the distance between two dates does not depend on which way you measure it.
- Why does the countdown change every day?
- The relative-to-today row reads the current date, so a page you bookmarked or shared last month still reports the correct number of days left rather than a stale figure. The other rows depend only on the two dates you entered and never change.
Use this with
Date Calculator answers one part of the question. These pick up where it stops: Days Until Calculator, Age Calculator, Hours Calculator and Pregnancy Due Date Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- ISO: ISO 8601 date and time format. The international standard for writing and ordering dates
- GOV.UK: UK bank holidays. Published non-working days used when converting calendar days to working days
- Fair Work Ombudsman: Public holidays. Australian public holidays that interrupt a business-day count
Date Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.