Baby Days Calculator

A baby days calculator turns a birth date into a parent-friendly age summary: total days since birth, complete weeks, months old, years-months-days, and the next fixed milestone. Parents often use it for growth notes, photo captions, family messages, baby journals, full-month and hundred-day celebrations, and everyday curiosity about how much time has passed since birth. Day counts are often easier to compare than calendar months, while month age is more familiar for feeding, sleep, and development discussions. This tool uses the date entered and today’s browser date; it does not replace medical guidance. Vaccination schedules, premature-baby corrected age, clinical milestones, and local child health rules should still be checked with a pediatrician or official clinic schedule.

All four outputs come from a single Date pair: birthDate parsed from the picker and today taken from new Date(). Total days is Math.floor((today - birthDate) / 86_400_000) — the constant is milliseconds per day, which is correct as long as both timestamps live in UTC. The DST trap is real: if the calculation is done in a local time zone that crossed a spring-forward boundary between the two dates, one of the days is 23 hours long and the naive subtraction undercounts by 1. Using Date.UTC(year, month, day) to anchor both endpoints sidesteps it entirely. Years/months/days is a component-wise subtraction with borrow: start from y2−y1, m2−m1, d2−d1; if the day component is negative borrow a month (and adjust by the number of days in the previous calendar month, 28–31 depending on which month it is and whether the year is a Gregorian leap year — divisible by 4, except centuries unless divisible by 400); if the month component is negative borrow a year. That algorithm matches how parents naturally describe age, so January 31 → February 28 reads as '0 months 28 days' rather than '1 month'. Milestones are fixed day counts that follow Chinese tradition (the 100-day banquet is day 99 by counting convention, day 100 in everyday speech) and Western anniversaries (365, 730, 1095 calendar days). The CDC Developmental Milestones (revised 2022) put first social smile around 6 weeks, sitting unaided near 6 months, first steps around 12 months, and the 50-word vocabulary near 18 months — but those are clinical benchmarks, not what this tool reports. Birth date never leaves the browser; nothing is persisted unless the parent explicitly bookmarks the URL with the date encoded.

• Total days = Math.floor((today − birthDate) / 86_400_000); use Date.UTC(y,m,d) for both endpoints to avoid the off-by-one when a DST transition falls between them
• Years/months/days is a component-wise subtraction with borrow — when the day component goes negative, borrow days from the previous calendar month (28–31 per Gregorian rules)
• Gregorian leap year rule: divisible by 4, except century years unless divisible by 400 (so 2000 is a leap year, 2100 is not); Feb 29 birthdays advance to Mar 1 in common years per GB/T 7408
• Weeks of life = Math.floor(totalDays / 7); months of life = years × 12 + months — these are the units pediatricians use for vaccination and growth charts
• Milestone day counts: 1 month = 30 days (calendar-month rollover), 100 days = day 99 by Chinese counting convention, half-year = 180, birthdays = 365·n
• CDC Developmental Milestones (2022 revision) place social smile ≈6 weeks, sitting unaided ≈6 months, first steps ≈12 months, 50-word vocabulary ≈18 months — useful context, not a clinical schedule
• Birth date stays in the browser; saving across sessions requires the parent to bookmark a URL with the date in the query string — the page itself writes nothing to localStorage