About DateSpan

DateSpan is a small collection of date and age calculators built to one standard: they have to be correct, fast, and private.

Most online date tools are close enough for a rough estimate and wrong at exactly the moments that matter — a leap-day birthday, a month-end borrow, the days around New Year. DateSpan exists because those edge cases are not edge cases for everyone. For a speech-language pathologist scoring a norm-referenced test, a one-month error in a child's age can change the result. We wanted a set of tools we would trust with that.

What we focus on

Our anchor tool is a chronological age calculator aimed at assessment professionals — SLPs, school psychologists, occupational therapists, and special educators — who need age in the years;months;days format their test manuals expect, computed with the correct borrowing rules and no rounding. Around it we build everyday calculators the same way: age in months and days, days and business days between dates, ISO and US week numbers, a weeks-until countdown, and a date add/subtract calculator.

How we build

• Correct by construction. Every calculation runs on plain local year-month-day values. We never route a date through a UTC timestamp, which is the usual cause of "off by one day" bugs.
• Tested. The date engine ships with a suite of automated tests covering leap years, month-end borrowing, cross-year week numbers, and holiday counting.
• Private. All processing happens in your browser. The dates you enter never leave your device — nothing is uploaded, and nothing is stored on a server.
• Fast and light. No frameworks, no trackers loading in the background, no layout that jumps around while an ad loads. Pages are small and work on any modern browser.

Honesty about limits

These are calendar tools, not professional advice. A chronological age from DateSpan should still be checked against your test manual's own instructions before you score; a business-day count reflects US federal holidays for 2024–2030 and may not match your employer's calendar. Where a convention is a judgement call — such as how to treat a February 29 birthday — we tell you which rule we use so you can document it. If you spot something that looks wrong, we want to hear about it; see the contact page.