A while back I told my landlord I would give him 60 days’ notice before moving out. I opened my calendar, counted forward on my fingers, circled a date and sent the message feeling very organised.
Two days later he replied politely: "That’s actually 59 days. Could you resend?" I had counted the day I wrote the message as day one, and he had not. One day. It did not cost me anything in the end, but it made me realise how easy it is to get a simple date count wrong, especially when real deadlines are involved.
Since then I have counted days for notice periods, invoice due dates, return windows, project deadlines and a few countdowns just for fun. I have also made most of the common mistakes along the way. This guide covers the three methods I actually use, how to handle business days properly, and the traps that catch almost everyone at least once.
Before you count anything: decide if the first day counts
This is the single biggest source of errors, so let’s sort it out first. When you count days between two dates, you have to decide whether to include the start day, the end day, both or neither.
Take January 1 to January 31. There are two reasonable answers:
- Exclusive count (the usual "difference"): 30 days. You are measuring how many days pass between the two dates.
- Inclusive count: 31 days. You are counting every calendar day from the 1st to the 31st, both ends included.
Neither is "wrong". They answer different questions. If you are asking "how long until my deadline?", you usually want the exclusive count. If you are asking "how many days will I be on holiday?" (leaving on the 1st and returning on the 31st), the inclusive count is more natural.
My own rule is simple. Before I count, I ask myself: who set this deadline, and how do they count? A landlord, a contract, a shop’s return policy or a visa office may each have their own rule. When the money or the legal consequences matter, I look for the exact wording instead of guessing.
Method 1: Counting by hand (and the month trick that makes it easy)
Counting by hand sounds old-fashioned, but it is still the fastest way for short distances, and it helps you sanity-check the results from any tool. I use it all the time when I only need a rough answer.
Step 1: Know your month lengths
You probably know the rhyme: thirty days has September, April, June and November. All the rest have thirty-one, except February, which has 28 (or 29 in a leap year).
If rhymes are not your thing, use the knuckle trick. Make a fist and look at the knuckles and the gaps between them. Start with the first knuckle as January. Every month that lands on a knuckle has 31 days, and every month that lands in a gap has 30 (February is the short gap). When you reach the last knuckle at July, go back to the first knuckle for August and carry on.
Step 2: Walk from month to month
Instead of counting every day one by one, jump to the same day in the next month, then add the leftover days. Here is a real example. Today is November 6, 2026, and I want to know how many days there are until Christmas Day, December 25, 2026.
- November 6 to December 6 is one full month. November has 30 days, so that is 30 days.
- December 6 to December 25 is 19 days (25 − 6).
- Total: 30 + 19 = 49 days.
Another way to check it: from November 6 to November 30 is 24 days, and from November 30 to December 25 is 25 days. 24 + 25 = 49. Same answer, which is what I want to see.
Step 3: Turn days into weeks if that helps
49 days is exactly 7 weeks. That also tells me something useful: Christmas Day 2026 falls on the same weekday as today. Today is a Friday, so Christmas is a Friday too. Dividing by 7 is a quick way to work out which day of the week a date lands on.
Another by-hand example: adding days to a date
Sometimes you do not have an end date. You have a start date and a number of days. Say a contract says something must happen within 90 days of January 15, 2027.
- January 15 to February 15 = 31 days
- February 15 to March 15 = 28 days (2027 is not a leap year), so the running total is 59
- March 15 to April 15 = 31 days, so the running total is 90
90 days after January 15, 2027 is April 15, 2027. Walking month by month like this is much less error-prone than counting 90 individual days.
Do not forget leap years
A leap year adds February 29, and it matters whenever your date range crosses the end of February. The rule is:
- A year divisible by 4 is a leap year (2028, 2032).
- But a year divisible by 100 is not (2100 will not be a leap year).
- Unless it is also divisible by 400 (2000 was a leap year).
For everyday date counting, "every four years" is right almost every time. Just check February when your range is long. 2026 and 2027 are both normal years, and the next leap year is 2028.
Method 2: Using a spreadsheet (Excel or Google Sheets)
When I have more than one or two dates to count, or when the range is long, I switch to a spreadsheet. Both Excel and Google Sheets store dates as numbers behind the scenes, which is why simple subtraction works.
The simplest way: just subtract
Put the start date in cell A2 and the end date in cell B2. In C2, type:
=B2-A2
If A2 is November 6, 2026 and B2 is December 25, 2026, you get 49. If the result shows up as a date like "02/18/1900", the cell has a date format. Change the format of that cell to "Number" and the 49 will appear.
Other formulas that do the same job
=DAYS(B2,A2)gives the same result, and you write the end date first. I mix this up all the time, so I usually stick with plain subtraction.=DATEDIF(A2,B2,"d")also returns days. It can do more, which I explain below.
Getting years, months and days with DATEDIF
If you want the gap as "1 month and 19 days" rather than "49 days", DATEDIF is the tool. The last part of the formula tells it what to measure:
"d"for total days"m"for complete months"y"for complete years
For November 6 to December 25, =DATEDIF(A2,B2,"m") returns 1, because only one full month has passed. DATEDIF also has an "md" option for the leftover days, but it is known to give odd results in some cases, so I avoid it. I work out the leftover days by subtracting instead.
One more thing: in Excel, DATEDIF does not appear in the formula suggestions when you start typing. It still works. It is just a bit hidden, which confuses people. Google Sheets handles it normally.
Business days with NETWORKDAYS
Here is where spreadsheets really shine. NETWORKDAYS counts only Monday to Friday between two dates:
=NETWORKDAYS(A2,B2)
For November 6 to December 25, 2026, that returns 36. Both the start and the end date are included, and the formula skips Saturdays and Sundays. A quick sanity check: 49 calendar days is 7 weeks, and 7 × 5 = 35 weekdays, plus the extra end date which is a Friday, so 36.
Note the difference from the plain subtraction. =B2-A2 gives 49 and does not include the start date. NETWORKDAYS includes both ends. That is exactly the "inclusive or exclusive" problem from earlier, hiding inside two formulas.
Adding holidays to the formula
Weekends are only half of the story. Public holidays are not business days either. In a separate area of your sheet (say E2:E4), list the holiday dates, then add that range to the formula:
=NETWORKDAYS(A2,B2,E2:E4)
If you list December 25 as a holiday, the Christmas example above drops from 36 to 35. Each holiday that falls on a weekday is removed from the count. If a holiday falls on a Saturday or Sunday, it makes no difference, because the weekend was already excluded.
Different weekends: NETWORKDAYS.INTL
Not every country has a Saturday and Sunday weekend. In some places the weekend is Friday and Saturday. For these cases, use NETWORKDAYS.INTL. It lets you choose which days count as the weekend. For example, a weekend code of 7 means Friday and Saturday are off:
=NETWORKDAYS.INTL(A2,B2,7,E2:E4)
It can also take a seven-character text pattern for custom schedules (1 means off, 0 means working, starting from Monday). So "0000011" is the normal Saturday and Sunday weekend. This is handy if you or your team work a six-day week or a shifted schedule.
Finding a date that is N business days ahead
The opposite question is also common: "I have 10 business days to reply. What is my deadline?" Use WORKDAY:
=WORKDAY(A2,10,E2:E4)
This returns the date that is 10 working days after the date in A2, skipping weekends and any holidays you listed. I use this one a lot for delivery estimates and for deadlines in client work.
Method 3: Using an online date calculator
The third option is the one I reach for when I am on my phone or I just want a fast answer with no setup. A good online calculator lets you pick two dates and instantly shows the number of days, weeks, months and sometimes business days.
On this site, the Days Between Dates Calculator does exactly that: it gives you the days, weeks, months and business days between any two dates. Your dates are processed in your browser, so nothing gets sent anywhere. Other well-known options include timeanddate.com, which has been around for years and also has a handy business-day calculator.
A few more quick tricks that need no tool at all:
- Type "days until 25 December" into Google, and it shows a countdown straight away.
- Ask your phone’s assistant, such as Siri or Google Assistant, how many days there are until a date.
- For repeating reminders, add the deadline to Google Calendar or the iPhone Calendar app and set a notification a week before.
Online tools are great, but I always use them with a quick mental check. If the tool says 49 days and my rough estimate says "about seven weeks", I am happy. If it says 94, I probably typed a date wrong (more on that below).
How business days really work (and why the answer changes by country)
Business days sound simple: Monday to Friday, no weekends. In practice there are three things that make them tricky.
- Public holidays differ by country, state and sometimes city. A day that is a normal working day in one place can be a holiday in another.
- Organisations have their own rules. A bank, a courier and a government office may all define business days a little differently. Some banks do not count Saturdays, while others do for certain services.
- The weekend is not the same everywhere. Some countries have a Friday and Saturday weekend instead.
A step-by-step manual example
Let’s count the business days from Monday, January 4, 2027 to Friday, January 22, 2027, with both days included.
- Step 1: Count the calendar days. January 4 to January 22 inclusive is 19 days.
- Step 2: Count full weeks. 19 days contains 2 full weeks (14 days). Each full week has 5 working days, so that is 10.
- Step 3: Count the leftover days. The remaining 5 days run from Monday, January 18 to Friday, January 22, and all five are weekdays. That adds 5.
- Step 4: Add them up. 10 + 5 = 15 weekdays.
- Step 5: Remove public holidays. In the United States, Monday, January 18, 2027 is Martin Luther King Jr. Day, a federal holiday. That leaves 14 business days.
In some German states, January 6 (Epiphany) is also a public holiday, so the answer there would depend on the state. Same dates, different answers. That is why I always ask "which holidays?" before I trust a business-day count.
The rough shortcut
If you only need an estimate, multiply the calendar days by 5/7. For 49 calendar days, 49 × 5 ÷ 7 = 35. It is not exact, because it depends on which weekday you start on, but it is close enough for a "roughly how many working days do I have?" question.
Real situations where counting days really matters
Here are the places I have seen date counting cause the most confusion, with the numbers worked out.
Return windows after shopping
Say you buy something on Friday, November 27, 2026 (Black Friday) and the shop offers a 30-day return window. Counting 30 days from the purchase date gives you December 27, 2026. That is a Sunday. If the shop is closed on Sundays, you effectively need to go on Saturday the 26th or the store might still accept a postal return, depending on its policy.
Two lessons here. First, always check which weekday the deadline lands on. Second, find out whether the window starts on the purchase date or the delivery date. For online orders in the EU, the usual 14-day right of withdrawal for distance purchases generally starts from the day you receive the goods, not the day you placed the order. If it matters, read the seller’s policy to be sure.
Invoice payment terms
If you have ever sent an invoice that says "net 30", the payment is due 30 days after the invoice date. An invoice dated November 6, 2026 is due on December 6, 2026, which happens to be a Sunday. In practice, a client might pay on the Friday before or the Monday after, so I add a note about the due date and keep a reminder a few days earlier. When I invoice clients, I also use the spreadsheet formula =A2+30 to calculate due dates. No mental maths, no mistakes.
Notice periods for jobs and rentals
This was my landlord story. Notice periods can be counted from the date you send the notice, the date it is received, or the first of the next month. Some contracts say "notice must be given by the end of the month". Read the exact wording and, if you are unsure, ask in writing. For anything with legal or financial weight, I check the document itself or ask the other party instead of relying on my own count.
Project deadlines and shipping estimates
"Delivered in 5 to 7 business days" is a classic. If you order on a Thursday, those business days skip the weekend, and any holiday in between pushes the date further. For a quick estimate, I use WORKDAY in a spreadsheet, and I assume at least one extra day if a holiday is near.
Personal countdowns
Birthdays, trips, exams, a wedding. Counting down is the fun use. From November 6, 2026 to January 1, 2027 is 56 days. It is a good reminder of how fast the end of the year comes. For counting your own age in years, months and days, the guide on how to calculate your exact age uses the same ideas.
Common mistakes to avoid
These are the ones I have made myself or seen others make.
- The off-by-one error. Forgetting whether the first or last day counts. Decide first, then count.
- Assuming every month has 30 days. A "month" can be 28, 29, 30 or 31 days. For long ranges, those differences add up.
- Mixing up date formats. 03/04/2026 means March 4 in the US but April 3 in most of Europe. A tool or sheet that reads the date the wrong way will give you a completely different answer. I type dates as 2026-03-04 (year first) when I want to be safe, because it cannot be misread.
- Dates stored as text in a spreadsheet. If a date is left-aligned and the formula returns an error or nonsense, it is probably text. Re-enter it or use the "Format as date" option.
- Forgetting holidays in a business-day count. Weekends are easy to skip, but holidays are easy to forget. List them in a range and include them in the formula.
- Assuming the weekend is the same everywhere. If you work with people in another country, check their working week.
- Ignoring leap years. If your range crosses February in a leap year, add the extra day.
- Using the wrong start date. Order date, shipping date, delivery date and invoice date are all different. Make sure you are counting from the one your deadline actually uses.
- Counting hours instead of days across clock changes. When the clocks change for daylight saving time, a "day" can be 23 or 25 hours long. If you only use dates and not times, this does not affect you.
- Trusting a rough estimate for something important. The 5/7 shortcut is fine for planning, but not for a legal or contractual deadline.
A small checklist I run through
Whenever a date count matters, I go through the same few questions. It takes less than a minute.
- Does the first day count? Does the last day count?
- Are these calendar days or business days?
- Which holidays apply, and which weekend pattern?
- Which weekday does the final date land on?
- Did I get the same answer from two different methods (for example, by hand and with a tool)?
If the answers to all five are clear, I send the message or set the reminder with confidence. If any of them is fuzzy, I find out before I commit to a date.
Quick answers to common questions
How do I find the number of days between two dates without a calculator?
Count the days month by month. Jump from the start date to the same day in the next month, add the days of each month you pass, and then add the leftover days. Check the result by counting the other way round.
What is the formula for days between two dates in Excel or Google Sheets?
Subtract the earlier date from the later one: =B2-A2. You can also use =DAYS(B2,A2). Make sure the result cell is formatted as a number.
How do I count only working days between two dates?
Use =NETWORKDAYS(A2,B2) in a spreadsheet, and add a holiday range as a third argument if you need it. By hand, count full weeks × 5 and then the leftover weekdays, and subtract public holidays.
Do I include the start date when counting days?
It depends on the context. A plain difference between two dates does not include the start day. A "from the 1st to the 31st" style count usually includes both ends. When it matters, check how the rule or contract defines it.
How many weeks are there in 49 days?
Exactly 7. Divide the number of days by 7, and any remainder is the extra days.
One last thing before you go
After the landlord incident, I started doing something very simple: whenever I count a deadline, I write the final date down together with the weekday, like "Friday, December 25". Seeing the weekday next to the date catches so many silly mistakes, like a deadline that lands on a Sunday or a date that is a week off.
You do not need to be a maths person for any of this. Walk month by month for short ranges, let a spreadsheet do the work for anything long or repeated, and use a calculator when you just want the answer fast. Whichever you choose, check the first-day question and the holiday question, and you will avoid almost every mistake I have made.
This article is for general information only. For legal, contractual or official deadlines, check the exact rules that apply to you or ask the relevant organisation.
This guide is for general information only. See our disclaimer.