About

Parental records require precise age notation. Pediatric dosing, vaccination schedules (CDC/WHO), and developmental assessments all depend on exact age in completed months and days, not rough approximations. A common error is treating every month as 30 days, which accumulates a drift of up to 5 days over six months. This tool computes calendar-accurate age using actual month lengths and leap year corrections. For premature infants, corrected age (subtracting weeks born early from chronological age) is critical until 24 - 36 months. Incorrect age reporting leads to misplotted growth percentiles and mistimed milestone flags.

The calculator derives total elapsed time from the birth timestamp t₀ to the current instant t_now, decomposing Δt = t_now − t₀ into calendar units. It tracks 12 developmental milestones from the 100-day mark through the 1000th day. Note: this tool assumes full-term birth unless corrected gestational age is specified. Time zone differences between birth location and current location are not compensated.

Formulas

The baby's chronological age is computed by calendar subtraction from birth date D_birth to today's date D_now. The algorithm walks month-by-month to respect variable month lengths.

Δt = D_now − D_birth

Total elapsed days are decomposed into calendar years, months, and remaining days:

Y = floor(Δt365.2425)

For precise month calculation, the algorithm counts completed calendar months from the birth day-of-month to the current day-of-month. If the current day is less than the birth day, the previous month is used as the boundary, and remaining days are counted from there.

Age_corrected = Age_{chronological} − (40 − GA) weeks

Where GA = gestational age at birth in weeks. Total time conversions use exact millisecond difference:

Total_days = floor(t_now − t₀86400000)

Total_weeks = floor(Total_days7)

Next birthday countdown: find the next occurrence of the birth month/day after D_now, compute Δ in days/hours/minutes.

Reference Data

Milestone	Age	Typical Development	Medical Note
100 Days	3 mo 10 d	Social smiling, head control	2-month vaccines due
6 Months	26 wk	Sitting with support, babbling	Introduce solid foods
9 Months	39 wk	Crawling, pincer grasp	Developmental screening
1 Year	365 d	First steps, first words	12-month vaccines
18 Months	78 wk	Walking well, 10-20 words	Autism screening (M-CHAT)
2 Years	730 d	Two-word phrases, running	Growth percentile check
1000 Days	2 yr 9 mo	Complex sentences	Critical nutrition window closes
3 Years	156 wk	Pedaling tricycle, questions	Vision screening
4 Years	208 wk	Counting, storytelling	4-year vaccines
5 Years	1826 d	Writing name, cooperative play	School readiness assessment
Half Birthday	6 mo intervals	Varies by age	Mid-year growth check
Leap Day Baby	Feb 29 birth	Legal birthday varies by jurisdiction	Age counted from actual date
Corrected Age	Chronological − weeks early	Used until 24 - 36 mo	Preemie milestone adjustment
Newborn Period	0 - 28 d	Reflexes, feeding patterns	Newborn screening tests
Infant Stage	1 - 12 mo	Rapid growth phase	Monthly well-baby visits
Toddler Stage	1 - 3 yr	Language explosion	Dental visit by age 1

Frequently Asked Questions

The algorithm does not assume 30-day months. It steps through each calendar month individually (28, 29, 30, or 31 days). If the baby was born on January 31 and today is March 2, it counts January 31→February 28/29 as 1 month, then February 28/29→March 2 as the remaining days. This matches how pediatricians count age.

Corrected age (also called adjusted age) subtracts the weeks of prematurity from chronological age. A baby born at 32 weeks gestational age (8 weeks early) who is chronologically 6 months old has a corrected age of 4 months. The AAP recommends using corrected age for developmental milestone assessment until 24 months for moderate preemies and 36 months for very preterm infants (born before 28 weeks).

The ticker uses requestAnimationFrame tied to Date.now() rather than a simple setInterval counter. Each frame recalculates elapsed milliseconds from the birth timestamp, so accumulated timing errors from frame drops or tab backgrounding are automatically corrected. The display updates at approximately 60 fps when the tab is active.

Babies born on February 29 have their age calculated normally using calendar arithmetic. In non-leap years, the birthday countdown targets March 1 by default (following the convention in most jurisdictions). The total day count remains exact regardless, as it is computed from millisecond timestamps. The age in years increments on March 1 in non-leap years.

Calendar months and weeks do not align neatly. A 3-month-old baby is not exactly 12 weeks old because months range from 28 to 31 days. For example, a baby born January 15 is 3 months old on April 15 (90 days in a non-leap year), which equals 12 weeks and 6 days. Pediatricians use weeks for the first 3 months, then switch to months.

Yes. If you provide a birth time, the calculator uses it for the hours/minutes/seconds breakdown and the live ticker. A baby born at 11:30 PM is only 30 minutes old at midnight, not a full day. If birth time is omitted, midnight (00:00) is assumed, which may overcount hours within the first day.