Fast. Accurate. Free.
Every date and time calculation, in one place
ReckonDay is a fast, accurate directory of date and time tools: age, days between dates, countdowns, business days, time zones, work hours, and more.
Browse by category
Age & Birthday Calculators
Exact age, age differences, birthday countdowns, milestones, and everything else built around the date you were born.
Date Math Calculators
Days between dates, adding or subtracting durations, business days, week numbers, and calendar arithmetic done exactly right.
Time & Duration Calculators
Work hours, time cards, duration across midnight, sleep timing, and adding or subtracting clock time.
Countdown Calculators
Live countdowns to any date or event, school-term countdowns, and other time-until tools.
Calendar Tools
Printable calendars, perpetual weekday lookups, and days-in-month/year reference tools.
Time Zone Calculators
Time zone conversion, multi-timezone meeting planning, world clock, and unix timestamp conversion — all via the browser's built-in IANA database.
Most-used tools
Reference libraries
Beyond the 38 interactive calculators, ReckonDay maintains six large curated reference families for common long-tail searches — every one browsable, not just sitemap-listed.
Why ReckonDay
Most date calculators get the easy cases right and the edge cases wrong — leap years, daylight saving transitions, month-end rollovers, and business-day rules with holidays are where the math actually gets hard. Adding one month to January 31st doesn't have one obvious right answer (does it clamp to February 28, or roll forward to March 3?), and a business-day count depends on a weekend definition that genuinely isn't the same everywhere in the world — most countries treat Saturday and Sunday as the weekend, but several, including much of the Middle East, use Friday and Saturday instead.
ReckonDay's calculators are built on one shared, unit-tested date-math core (see the Methodology page for the underlying test suite) so every tool on the site handles these edge cases the same, correct way — a leap year is determined identically whether you're checking February's day count, a year's total day count, or a birthday's weekday-shift pattern, rather than each tool implementing its own slightly different, hand-rolled approximation. Every calculator ships with genuine, specific explanatory content — the method, a worked example, and real edge cases for that exact tool — rather than a bare input box with no context.
The site is organized into six categories — Age & Birthday, Date Math, Time & Duration, Countdowns, Calendars, and Time Zones — covering 38 individual calculators in total, plus a curated set of pre-built countdown and reference pages for commonly searched dates and public holidays across 20 countries.
By the numbers
38
calculators
6
categories
125
countdown pages
20
countries covered
Frequently asked questions
Is ReckonDay free to use?
Yes — every calculator on the site is free, with no account or sign-up required. There's an optional support page for anyone who wants to contribute toward hosting costs.
Does ReckonDay store the dates I enter?
No — every calculation runs entirely in your browser using your device's own clock and the browser's built-in time zone database. Nothing you enter is sent to a server to compute a result. See the Privacy page for the full policy.
Why do the tools handle leap years and DST differently from other calculators I've used?
Most of the disagreements between date calculators come down to an unstated convention — inclusive vs. exclusive day counting, clamping vs. rolling over at a month's end, or a different weekend definition. Every tool on this site states its convention explicitly in its own how-it-works section rather than leaving you to guess.
Which calculator should I start with?
The Age Calculator, Days Between Dates, and Date Plus or Minus Days are the three most commonly used tools on the site and a good starting point for most everyday date-math questions.
Does the site work the same way on mobile as on desktop?
Yes — every calculator runs the identical client-side date-math engine regardless of device, so a result computed on a phone matches the same calculation run on a desktop browser exactly, since both read the same underlying time zone and calendar data built into the browser itself.
Can I use ReckonDay's tools for a country other than the US?
Yes — the underlying calendar and time zone math is universal, and country-specific details (weekend definitions, public holidays) are explicitly adjustable rather than hardcoded to one country, with dedicated per-country holiday pages covering 20 countries.