Calculation Methodology & Edge Cases

• Complete units only. A year or month is counted only when it is fully complete. Whatever is left over is expressed in the next smaller unit, ending in days.
• Month-end clamping. When a monthly anniversary does not exist in a shorter month, it is observed on the last day of that month. For example, the one-month anniversary of January 31 is February 28 (or February 29 in a leap year). So January 31 to March 1 is 1 month, 1 day — not a negative or fractional result.
• Leap-day birthdays. A person born on February 29 has their birthday observed on February 28 in non-leap years. February 29, 2024 to February 28, 2025 is exactly 1 year.
• Midnight normalization. All dates are compared at midnight local time, so the time of day never changes a result.
• Totals are exact. Total days, weeks, hours, and minutes are computed from the exact number of midnights between the two dates, so leap years are always counted correctly (2020-01-01 to 2021-01-01 is 366 days).

The same chronological age is recorded four different ways depending on the instrument. AgeCalc computes all four from one pair of dates and never guesses which one you need:

The 15-day threshold is the published BRIGANCE rule and the common convention wherever an instrument asks for age rounded to the nearest month rather than complete months. Truncation is the default everywhere else — including the Wechsler-family and Pearson language batteries, where the norm band is selected by the complete years and months. Applying the wrong convention shifts the examinee into a neighbouring band, which is why both figures are always shown together.

AgeCalc produces every format on the assessment age calculator, the Brigance age calculator (rounded), the Pearson age calculator (years;months.days), the age in months calculator, and the per-instrument pages listed in the instrument reference.

Corrected age for premature babies is the age the child would be if born on their due date:

• Corrected age = assessment date − due date
• Weeks premature = (due date − birth date) ÷ 7
• Equivalently: corrected age = chronological age − weeks premature.

If the assessment date falls before the due date, corrected age is negative and reported as 0. Following guidance from the American Academy of Pediatrics (HealthyChildren.org), correction is typically used until the child's second birthday, and only for developmental milestones and growth — never for vaccination schedules or medication dosing, which always use chronological age. See the adjusted age calculator for the interactive version and sources.

A due date is, by definition, 40 weeks 0 days of gestation. Every gestational figure on the site follows from that single assumption:

• Gestational age at birth = 280 days − (due date − birth date), expressed as completed weeks plus days (e.g. 32w 4d).
• Postmenstrual age = gestational age at birth + chronological age. This is the age used while a baby is still in neonatal care.
• Entering a gestational age instead of a due date simply runs the first formula backwards, so both routes give identical results.

If a due date implies a gestation outside 20–45 weeks, the weeks figures are suppressed rather than reported, because the input is almost certainly a typo. Corrected age is still shown, since it depends only on the due date.

The calculation functions are covered by automated unit tests that run on every change, including:

• Month ends: Jan 31 → Feb 28/29, Mar 31 → Apr 30, Dec 31 → Jan 30
• Leap days: Feb 29 birthdays across leap and non-leap years
• The years;months.days string, and nearest-month rounding either side of the 15-day threshold (14, 15 and 16 day remainders, and the year rollover at 11;15)
• The BRIGANCE prematurity adjustment and the Data Sheet band it resolves to
• Future dates and same-day dates (result is zero, never negative)
• Time-of-day boundaries (11:59 PM vs midnight)
• Due date before, on, and after the birth date (preterm, term, post-term)
• Corrected age before the due date (reported as 0)
• The second-birthday boundary where correction typically stops
• Gestational and postmenstrual age, including implausible due dates and agreement between the due-date and gestational-age input routes
• Korean counting age, year age, and international age at every point in the year

Pages that make health-adjacent or assessment-related claims show a visible review panel with sources and a last-reviewed date. Content is checked against primary sources — publisher documentation for assessment conventions, and bodies such as the American Academy of Pediatrics (HealthyChildren.org) for corrected-age guidance — before publication and after any change to the calculation logic. No clinician is credited as a named reviewer, because none has reviewed this material; see about for who publishes the site and why it says so plainly. AgeCalc calculators are educational tools and are not affiliated with Brigance, Pearson, or any other assessment publisher.

Found a calculation you believe is wrong? The conventions above are the contract — if a result contradicts them, it is a bug. Reach us via the about page.