About these tools and how they are checked

What this site is

DoubleCheckCalc is a small set of free date and time calculators. Each page does one job, opens with a worked example and explains in plain words how the answer is reached, including the awkward cases. The site will be paid for by advertising; no ads are shown yet. There is no sign-up, and nothing you type leaves your browser (see privacy).

Operated by Longitude Press (sole trader), ABN 38 842 876 454, NSW.

How the date maths is done

• Dates are handled as whole calendar days pinned to UTC, so a daylight saving change can never add or drop a day.
• Adding months clamps to the end of a short month: 31 Jan plus 1 month is 28 Feb, or 29 Feb in a leap year.
• Ages treat a 29 February birthday as 1 March in years without a leap day.
• Leap years follow the Gregorian rule: every fourth year, except century years that do not divide by 400.
• Percentages, fractions, ratios, averages and the scientific calculator keep every number as an exact fraction, so 0.1 + 0.2 is exactly 0.3 and 1 ÷ 3 × 3 is exactly 1.
• Answers that cannot be exact (most square roots, sines, logs and anything with π) are worked out in the browser's floating point and rounded to 12 significant digits on screen; they are marked with ≈.
• The scientific calculator follows the usual order: brackets, powers, then × and ÷ from left to right, then + and − from left to right. It shows how it read your sum, with brackets added.
• Random numbers come from the browser's cryptographic source (crypto.getRandomValues), and draws that would favour some numbers are thrown away and drawn again.
• The percentage, math, statistics and random tools use a second script, mm.js, which you are also welcome to read.
• All of it lives in one small script, tt.js, which you are welcome to read.

How the tools are checked

Before the site is published, an automated check opens every page in a real browser (Chromium) and types in questions with known answers. The published pages have to give those answers. Some of them:

• 1 Jan 2026 to 31 Dec 2026 is 364 days, or 365 counting the end date.
• 31 Jan 2026 plus 1 month is 28 Feb 2026; 31 Jan 2024 plus 1 month is 29 Feb 2024.
• Friday 13 Nov 2026 to Monday 16 Nov 2026 is 2 working days.
• Someone born 29 Feb 2024 is 0 on 28 Feb 2025 and 1 on 1 Mar 2025.
• 22:00 to 06:00 is 8 hours; 9:00 AM to 5:30 PM less a 30 minute break is 8 hours.
• 1:30:00 plus 0:45:30 minus 0:15:00 is 2:00:30.
• 0.1 + 0.2 = 0.3, 2 + 3 × 4 = 14, 2^3^2 = 512, −2² = −4, sin(30°) = 0.5 and tan(90°) is refused.
• 15% of 80 is 12; 45 is 90% of 50; 80 to 100 is a 25% increase.
• 1/2 + 1/3 = 5/6; 12:18 simplifies to 2:3; the GCF of 48 and 36 is 12 and their LCM is 144.
• The standard deviation of 2, 4, 4, 4, 5, 5, 7, 9 is 2 for a population.
• A random number from 1 to 6, drawn 60,000 times in the unit tests, lands on each face close to 10,000 times.

The same check confirms that no page reports a script error, that every page fits a 360 pixel wide phone screen without sideways scrolling, and that every internal link leads somewhere. The functions in tt.js are also tested on their own, outside the browser.

What has not been checked

• The automated check runs in Chromium only. Other browsers and screen readers have not been tested.
• No public holiday data is included anywhere on the site.

Found a mistake?

Email [email protected] with the page, what you typed and what you expected. Confirmed errors are fixed and added to the check.