How Many Days Between Two Dates? (With Formula & Examples)

Whether you're counting down to a vacation, figuring out how many days remain on a contract, calculating how long ago something happened, or just satisfying your curiosity — knowing how many days are between two dates is a surprisingly useful skill. This guide explains the formula, walks through manual examples, covers the difference between calendar days and business days, and shows you how to do it without a calculator if you ever need to.

How to calculate days between two dates: the basic formula

At its core, the formula is simple:

Days between dates = End date − Start date

The trick is expressing both dates as numbers so you can subtract them. Every date can be converted to a "serial number" — a count of days elapsed since some fixed reference point. Spreadsheets use January 1, 1900 as their reference (so January 2, 1900 = serial number 2). Unix systems use January 1, 1970 as their epoch. Whichever reference point you use, the math is the same: subtract the smaller number from the larger.

You don't need to work in serial numbers by hand. If you use a date difference calculator, you enter the two dates and get the answer instantly. But understanding the underlying logic helps when you need to sanity-check a result or do rough mental math.

Manual method: how to count days between two dates by hand

For dates within the same year, the easiest approach is to count through the months:

Step 1: Count full months between the dates

Start with the month immediately after your start date and count each complete calendar month up to (but not including) the month of your end date. Add up the days in each of those months.

Step 2: Add the remaining days

Add the days remaining in the start month (from the start date to the end of that month) and the days elapsed in the end month (from the 1st to the end date).

For dates that span multiple years, add the days remaining in the start year (from your start date to December 31), multiply any complete intervening years by 365 (adding 1 for each leap year that falls in the range), then add the days elapsed in the end year (from January 1 to your end date).

Calendar days vs. business days: what's the difference?

Calendar days count every single day — Mondays through Sundays, holidays included. If you count 30 calendar days from today, you're counting every day on the calendar page.

Business days (also called working days) count only Monday through Friday, excluding recognized public holidays. If your employer says you have 10 business days to respond to something, that's two full work weeks — but it could span three calendar weeks if weekends and a holiday fall in between.

The distinction matters more than most people realize:

• Legal and contract deadlines are often stated in business days. A "30-day right of rescission" on a real estate contract may actually mean 30 calendar days, while a "10 business day" payment term means two work weeks.
• IRS deadlines — If a tax deadline falls on a Saturday, Sunday, or legal holiday, the IRS automatically pushes the deadline to the next business day.
• Loan and mortgage timing — The Truth in Lending Act (TILA) requires lenders to give borrowers at least three business days to review closing disclosures, not three calendar days.
• Shipping estimates — "5 business day shipping" means the package won't arrive for a full calendar week if ordered late on Friday.

How to count business days manually

The formula is:

Business days = Calendar days − Weekend days − Public holidays

To count the weekend days in a date range, divide the total calendar days by 7 to get full weeks (each containing 2 weekend days), then check whether any partial week at the start or end adds additional Saturdays or Sundays. Subtract those, then subtract any public holidays that fall on weekdays within your date range.

Converting days to weeks and months

Once you have the number of calendar days, converting to other units is straightforward:

• Weeks: Divide days by 7. Remainder days are left over (e.g., 111 days ÷ 7 = 15 weeks and 6 days)
• Months: This is less precise because months vary in length. A rough estimate is days ÷ 30.44 (the average month length). For an exact answer — "3 months and 4 days" — you need to compare the calendar dates directly rather than dividing a day count.
• Years: Divide days by 365.25 to account for leap years. For a clean "X years, Y months, Z days" breakdown, use a calendar-aware calculator.

The date difference calculator returns all of these at once — calendar days, business days, weeks, months, and years — so you don't have to convert manually.

Leap years and why they matter

A year is a leap year if it is divisible by 4, except that century years (1800, 1900, 2000, 2100…) must also be divisible by 400. So 2000 was a leap year, but 1900 was not.

Leap years add an extra day (February 29) to the calendar. If your date range crosses February in a leap year, add 1 to your day count. For long ranges spanning multiple years, count the number of leap years in the interval and add that many extra days.

For most everyday calculations — how long until a vacation, how many days since a birthday — leap years won't change your answer by much. But for precise legal or financial calculations spanning years, they matter.

Common use cases for counting days between dates

Tracking how old you are in days

Most people know their age in years, but rarely in days. Someone who is 35 years old has lived roughly 12,775 days — that number tends to put time in a different perspective. Our age calculator shows your exact age in years, months, days, and even hours, and counts down to your next birthday.

Pregnancy and due dates

A full-term pregnancy is 280 days (40 weeks) from the first day of the last menstrual period. Obstetricians and midwives track gestational age in weeks and days — "you're 28 weeks and 3 days" — which is simply a date difference from the LMP date. See our pregnancy due date guide for the full breakdown of how gestational age is measured and how ultrasound can revise early estimates.

Loan terms and payoff dates

A 30-year mortgage is 10,957 days (or 10,958 in a period containing 8 leap years). Knowing the exact date your loan ends is useful for planning — especially if you're making extra payments to pay off earlier. Our date difference calculator lets you confirm what date a given number of days from now will be, as well as the reverse.

Event countdowns

Need to know exactly how many days until a wedding, product launch, or deadline? A countdown by calendar days is the most intuitive measure — it's what people mean when they say "only 47 days until the conference." A countdown calculator lets you set any future date and see the live count of days, hours, and minutes remaining.

Quick reference: days in each month

To do date math by hand, you need the days-per-month table:

• January: 31 days
• February: 28 days (29 in a leap year)
• March: 31 days
• April: 30 days
• May: 31 days
• June: 30 days
• July: 31 days
• August: 31 days
• September: 30 days
• October: 31 days
• November: 30 days
• December: 31 days

A useful memory device: the knuckle method. Make a fist and count across the knuckles (high points = 31 days) and valleys (low points = 30 or 28 days) starting from the index-finger knuckle as January. When you reach the pinky knuckle (July), start over from the index knuckle for August. Every knuckle is 31 days; every valley is 30 (or 28 for February).

The fastest way to calculate days between dates

For quick, accurate results without any manual counting, use a dedicated date difference calculator. Enter the start date and end date, and it instantly returns the gap in calendar days, business days, weeks, months, and years. No serial numbers, no counting knuckles, no leap-year math required.