How to Calculate Number of Days Between Two Dates (Every Method Explained)

Need to know how many days are between two dates? Whether you’re counting down to a deadline, calculating contract durations, figuring out someone’s age in days, or managing payroll, this guide covers every method: mental math, formulas, spreadsheets, and code.

The Fastest Method: Simple Subtraction

If both dates are in the same month, just subtract:

Example: How many days are between June 5 and June 23?

23 − 5 = 18 days

For dates that span multiple months, you need a slightly different approach.

How to Calculate Days Between Two Dates by Hand

Method 1: Count Through the Calendar

This works well for short date ranges.

1. Start on your first date.
2. Count forward day by day (or week by week) until you reach the second date.
3. The total count is the number of days between the two dates.

Example: March 28 to April 10

• March 28 → March 31 = 3 days
• April 1 → April 10 = 10 days
• Total = 13 days

Method 2: Convert to Day Numbers (Julian Day Method)

For larger date ranges, convert each date to a sequential day number within the year, then subtract.

Step 1: Find the day-of-year number for each date using this table:

For a leap year, add 1 to every month from March onward.

Step 2: Add the day of the month to get the day-of-year number.

Step 3: Subtract the two day-of-year numbers.

Example: How many days are between February 14 and November 3?

• February 14: 31 + 14 = day 45
• November 3: 304 + 3 = day 307
• 307 − 45 = 262 days

If you find it difficult to do it manually, use our free Julian date calculator to automate the whole process.

Method 3: Spanning Multiple Years

When the two dates fall in different years, use this approach:

1. Count the remaining days from the start date to the end of the year.
2. Add the full days for any complete years in between (365 or 366 for leap years).
3. Add the days from the start of the final year to the end date.

Example: How many days are between October 15, 2022 and March 4, 2024?

• Oct 15 to Dec 31, 2022: 77 days
• All of 2023: 365 days
• Jan 1 to Mar 4, 2024 (leap year): 64 days
• Total: 77 + 365 + 64 = 506 days

How to Calculate Days Between Two Dates in Excel (and Google Sheets)

Spreadsheets make this instant. Here are the most useful formulas.

Simple Subtraction Formula

=B1 - A1

Where A1 is the start date and B1 is the end date. Make sure both cells are formatted as dates. The result cell should be formatted as a Number, not a date.

DAYS Function

=DAYS(end_date, start_date)

Example:

=DAYS("2025-12-31", "2025-01-01")

Returns 364.

DATEDIF Function (Most Versatile)

DATEDIF is a legacy Excel function that calculates the difference in days, months, or years.

=DATEDIF(start_date, end_date, unit)

Example:

=DATEDIF("2023-06-01", "2025-09-15", "D")

Returns 836.

NETWORKDAYS Function (Business Days Only)

If you only want to count working days (excluding weekends and optionally holidays):

=NETWORKDAYS(start_date, end_date, [holidays])

Example:

=NETWORKDAYS("2025-01-01", "2025-12-31", H1:H11)

Where H1:H11 contains a list of public holidays to exclude.

Google Sheets supports all of these functions with identical syntax.

How to Calculate Days Between Two Dates in Python

Method 1: Using datetime (Built-in)

from datetime import date

start = date(2024, 3, 15)
end = date(2025, 9, 28)

delta = end - start
print(delta.days)  # Output: 562

The subtraction of two date objects returns a timedelta object. The .days attribute gives the total number of days.

Method 2: From String Inputs

from datetime import datetime

start_str = "2024-03-15"
end_str = "2025-09-28"

start = datetime.strptime(start_str, "%Y-%m-%d")
end = datetime.strptime(end_str, "%Y-%m-%d")

delta = end - start
print(abs(delta.days))  # abs() handles reverse-order inputs

Method 3: Using pandas (for Data Pipelines)

import pandas as pd

start = pd.Timestamp("2024-03-15")
end = pd.Timestamp("2025-09-28")

print((end - start).days)  # Output: 562

How to Calculate Days Between Two Dates in JavaScript

const start = new Date("2024-03-15");
const end = new Date("2025-09-28");

const msPerDay = 1000 * 60 * 60 * 24;
const daysBetween = Math.round((end - start) / msPerDay);

console.log(daysBetween); // Output: 562

Use Math.round() rather than Math.floor() to handle daylight saving time transitions correctly — DST can shift a day by ±1 hour, causing a floor division to produce an off-by-one error.

How to Calculate Days Between Two Dates in Other Tools

SQL

-- MySQL / MariaDB
SELECT DATEDIFF('2025-09-28', '2024-03-15');  -- Returns 562

-- PostgreSQL
SELECT '2025-09-28'::date - '2024-03-15'::date;  -- Returns 562

-- SQL Server
SELECT DATEDIFF(day, '2024-03-15', '2025-09-28');  -- Returns 562

PHP

$start = new DateTime("2024-03-15");
$end   = new DateTime("2025-09-28");
$diff  = $start->diff($end);

echo $diff->days;  // Output: 562

R

start <- as.Date("2024-03-15")
end   <- as.Date("2025-09-28")

as.numeric(end - start)  # Output: 562

Common Mistakes to Avoid

1. Including or Excluding the End Date

A recurring source of confusion: is the end date itself counted?

• Inclusive counting (counting both endpoints): used in everyday speech, e.g., “a 5-day holiday from Monday to Friday.”
• Exclusive counting (not counting the end date): used in most programming languages and spreadsheet formulas.

Rule of thumb: When calculating durations (how long something lasts), use inclusive counting. When calculating elapsed time (how many days have passed), use exclusive counting.

2. Ignoring Leap Years

Leap years add one day to February. If your date range spans February 29, your manual calculation must account for it. All programming methods handle this automatically.

3. Daylight Saving Time (DST) Errors in Code

When using timestamps (rather than pure dates) in JavaScript or Python, a DST transition can make a day appear to have 23 or 25 hours. Always work with date objects (not datetime or timestamp) when counting calendar days.

4. Time Zone Mismatches

If your start and end dates are stored in different time zones, converting to UTC before calculating prevents off-by-one errors. This is especially important in databases and APIs that store timestamps with timezone offsets.

Calculating Business Days Between Two Dates

To count only working days (Monday–Friday, excluding holidays):

In Excel / Google Sheets

Use =NETWORKDAYS(start, end, holidays) as shown above.

In Python

import numpy as np

start = np.datetime64("2024-03-15")
end   = np.datetime64("2025-09-28")

# Count business days (Mon–Fri), excluding no holidays
business_days = np.busday_count(start, end)
print(business_days)  # Output: ~402

# With custom holidays
holidays = ["2024-12-25", "2025-01-01"]
business_days = np.busday_count(start, end, holidays=holidays)

Real-World Examples

Example 1: Contract Duration

A freelance contract runs from April 1, 2025 to September 30, 2025. How many days?

• April: 30 days (remaining after April 1: 29)
• May: 31, June: 30, July: 31, August: 31, September: 30
• Total: 182 days (183 inclusive)

Example 2: Age in Days

Someone born on July 20, 1990 — how old are they in days on July 20, 2025?

• 35 years × 365 days = 12,775
• Plus 9 leap years (1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020, 2024) = 9 extra days
• Total: 12,784 days

Example 3: Days Until a Deadline

Today is May 1, 2025. Deadline is August 15, 2025.

• May: 30 remaining days
• June: 30, July: 31
• August: 15
• Total: 106 days

Frequently Asked Questions

Summary: Which Method Should You Use?

The core principle across every method is the same: convert both dates to a common numeric scale, then subtract. Whether you’re doing it by hand, in a spreadsheet, or in code, that’s all that’s happening under the hood.