The ten differences at a glance
| Difference | Example | Typical gap |
|---|
| Inclusive or exclusive count | March 6 to 14: 8 or 9 | 1 day |
| Month ends | January 31 plus a month: February 28 or March 3 | Up to 3 days |
| Months and days breakdown | January 31 to March 1: 1 month 1 day, or 29 days | Same total |
| UTC parsing | "2026-03-01" shown as February 28 | 1 day |
| Daylight saving | 47 hours read as 1 day | 1 day |
| February 29 birthdays | Age 22 or 21 on February 28 | 1 day of age |
| Excel's 1900 leap day | Spans across March 1, 1900 | 1 day |
| Calendar changes | British dates before September 1752 | 11 days |
| Skipped local days | Samoa, December 30, 2011 | 1 day |
| Holiday lists | US federal or stock market holidays | Several working days a year |
1. The first day counts, or it does not
From March 6 to March 14 is 8 days if you count gaps and 9 if you count both ends. A tool that reports 9 is not wrong. It answers "how many dates," not "how many days between." This calculator shows both.
2. Month ends
What is one month after January 31, 2026? This calculator says February 28, capping at the end of the shorter month. JavaScript's setMonth rolls over to March 3. Both are consistent rules. Only one matches what most people mean.
3. Splitting a span into months and days
January 31 to March 1, 2026 is 29 days. This calculator reads it as 1 month and 1 day: one capped month to February 28, then one more day. Another tool can reasonably say 29 days and no full month, since February 31 does not exist. The total days agree. The breakdown does not.
4. Dates parsed as UTC
In JavaScript, new Date("2026-03-01") means midnight UTC. In New York that is 7 p.m. on February 28, so a page that shows the local date displays the wrong day. This calculator builds dates from year, month, and day numbers to avoid that shift.
5. Daylight saving time
From noon March 7 to noon March 9, 2026 in New York is 47 hours. A tool that divides milliseconds by 24 hours and drops the remainder reports 1 day. Counting calendar dates gives 2.
6. February 29 birthdays
Someone born February 29, 2004 is 22 on February 28, 2026 under one convention and still 21 under the other. Tools pick one. This calculator uses February 28 and shows the March 1 reading next to it. The leap year guide has the details.
7. Excel's 1900 leap day
Excel treats 1900 as a leap year so that its date numbers match Lotus 1-2-3. February 29, 1900 exists in Excel and nowhere else. Any span that starts before March 1, 1900 and ends after it is one day too long in Excel.
8. Calendar changes
Britain and its colonies switched from the Julian to the Gregorian calendar in 1752: Wednesday, September 2 was followed by Thursday, September 14. George Washington was born on February 11, 1731 by the calendar of the time, which is February 22, 1732 in today's calendar. This calculator, like most software, applies the Gregorian calendar to all dates. For a historical date, convert it to Gregorian first.
9. Days a country skipped
Samoa moved across the International Date Line at the end of 2011, and December 30, 2011 did not happen there. A calendar count from December 29 to December 31, 2011 gives 2 days. Someone in Samoa lived 1. Calendar calculators count calendar dates, so they cannot see local skips like this.
10. Holiday lists
Working-day tools disagree whenever their holiday lists differ. A US tool may follow federal holidays or the stock market's. A UK tool may cover England and Wales or Scotland. This calculator names every holiday it removes, so you can see which list it used. The working days guide covers the differences.
How to settle a disagreement
- Compare the total days first. If those match, the tools differ only in how they display a breakdown.
- If the totals differ by one, check whether one tool counts both ends.
- If they differ by more, check the dates themselves: a swapped month and day, or a date before 1900.
- For a rule with consequences, such as a deadline or age limit, use the counting method the rule states. The calculator's methodology lists every rule it follows.
