Does the Day of Birth Count as Day 1?

Short answer: the day of birth is day 0

Medical and everyday age counting treats the day of birth as day 0. At the moment of birth the baby is 0 days, 0 months and 0 years old, and turns 1 day old the following day. Milestone traditions around the world often count the same day as day 1, because they mark which day of life it is rather than how much time has passed. Both answers are correct in their own contexts; the table below shows which system applies where.

Quick reference: which counting system to use when

What you are doing	Which system	What to say
Vaccinations, well-child checkups	Medical convention (birth day = day 0)	Give the birth date and let the doctor work out the difference
Plotting growth charts, comparing percentiles	Medical convention	WHO and CDC axes are both in completed days and months
Telling the doctor how old the baby is	Medical convention	When unsure, give the birth date itself
Everyday “how many days old is she?”	Elapsed-time counting (day 0)	The birth day is day 0, the next day is day 1
Hosting a one-month celebration	Family custom, usually the same date next month	Born March 5, one month on April 5
Hosting a 100-day celebration	Traditional ordinal (birth day = day 1)	The 100th day is about 99 full days after birth
Paperwork: IDs, school enrollment	Chronological age (0 years at birth)	Add 1 year on each birthday
When elders ask the traditional age	Traditional reckoning (1 year old at birth)	Add 1 year at each Lunar New Year

Three ways of counting a baby’s age

The confusion usually comes from three different ways of counting being mixed together:

1. Elapsed time (the everyday and medical convention): the question is how long the baby has been alive. At the moment of birth the baby has not completed a full day, so it is 0 days, 0 months, 0 years old. This is the system behind “how old is your baby” in everyday English, and behind vaccines, growth charts and medical records.
2. Ordinal day numbers (day-of-life counting): the question is which day this is. The day of birth is itself “day 1,” the way floors in a building are numbered from 1 rather than 0. Hospital charts that write “day of life 3” and celebrations such as the 100-day feast use this count.
3. Years lived through (traditional age reckoning): this counts how many calendar or lunar years a person has lived through, so a newborn is already 1 year old and gains another year at each new year. It answers a different question from day or month age and cannot be converted into them.

The same baby can be “0 days old” on a vaccination record, “day 1” in a hospital chart, “the 100th day” on a celebration invitation, and “1 year old” in a grandmother’s telling, all at once, because each number answers a different question.

The medical convention: the birth day is day 0

Pediatrics, obstetrics, vaccination schedules and growth monitoring all use a date-difference calculation: day age = today’s date minus birth date. By this convention:

• A baby born on the 1st is 0 days old on the day of birth
• The 2nd is 1 day old
• The 31st of the same month is 30 days old
• From the 1st of the next month, month age becomes 1 month (month age follows the calendar, not a fixed 30 days)

One wrinkle: some hospital records number the “day of life” starting at 1, so the birth day is written down as “day of life 1” even though the elapsed age is 0 days. That is an ordinal label, not a different birthday. The practical advice already covers it: give the exact birth date and let the clinician convert.

A few points that are easy to mix up:

• The neonatal period is the first 28 days after birth, that is, days 0-27.
• Weeks work the same way: the baby is 1 week old once 7 full days have passed (day-age 7), and 2 weeks old at day-age 14.
• Weeks and months are not interchangeable: 4 weeks is exactly 28 days, while 1 month runs from one calendar date to the same date the next month. An 8-week-old baby (day-age 56) is not necessarily “2 months old” yet, and vaccine schedules count in calendar months.
• Month age follows “the same date in the following month.” A baby born on May 5 turns 1 month old on June 5. At a month end: a baby born on January 31 turns 1 month old on February 28 in a common year, or February 29 in a leap year.
• Some newborn charts count in even finer units: jaundice phototherapy thresholds are plotted against age in hours, because a 24-hour-old baby and a 60-hour-old baby have different treatment cutoffs (AAFP · Hyperbilirubinemia in newborns).
• Why the world counts this way: the date difference is precise, verifiable and consistent everywhere. The WHO child growth standards and CDC growth charts mark their age axis in completed days and months, vaccine schedules are built on the same day-0 convention, and growth-chart apps follow it too. Pediatric dosing is a partial exception: doses are worked out mainly by weight, and day age matters most in the newborn period (WHO child growth standards, CDC growth charts).

For how this convention plays out at a real appointment, including growth charts and the questions a pediatrician asks, see what to log before a pediatrician visit.

Premature babies: corrected age from the due date

Preterm babies do not count month age from the birth date alone. Doctors use corrected age (also called adjusted age): take the time since birth and subtract the number of weeks the baby was born early. A baby born 8 weeks early who has actually been alive for 4 months has a corrected age of about 2 months, and that is the age to compare against growth curves and developmental milestones. Pediatricians generally keep using corrected age until about age 2, because most children have caught up to the typical milestone range by then (HealthyChildren.org · Corrected Age for Preemies). At an appointment, saying “the baby was born X weeks early” is more useful than offering a number that is easy to misread.

Milestone traditions and age reckoning around the world

Once the medical answer is clear, the remaining question is why so many celebrations seem to count “wrong.” They are not wrong: milestone traditions mark the ordinal day of life, so they count the birth day as day 1, and a few cultures have historically reckoned age itself differently. None of this affects medical records, but it explains the numbers on invitations and in family conversations.

China: the full-month and 100-day celebrations

• Full-month celebration: most commonly held on the same date in the following month, so a baby born on March 5 celebrates on April 5. Some regions count the birth day as day 1 and hold it at 30 days. Both are common, so follow the family’s custom.
• 100 days: traditionally the birth day counts as day 1, and the celebration falls on the 100th day, about 99 full days after birth, or roughly 3 months and 10 days. “One hundred” stands for completeness and marks the baby having passed the period when infant mortality was historically high.

Korea: Baek-il, the 100-day celebration

Korea’s 100-day celebration is called Baek-il (백일), literally “100 days,” and it is held on the 100th day after birth (Asia Society Korea · Baek-il). In earlier centuries infant disease was common and survival rates were low, so parents waited until the baby had safely passed the first 100 days before celebrating formally, offering rice and seaweed soup to the deity of childbirth in thanks. Modern celebrations are largely secular, but the 100th day still matters, followed later by a larger first-birthday party (Doljanchi).

Japan: Okuizome, the first-feeding ceremony

Japan’s Okuizome (お食い初め), also called momoka-iwai, is a first-feeding ceremony held roughly 100-120 days after birth (Hare-kura · Okuizome guide). An elder lifts food with chopsticks and pretends to feed the baby, with the wish that the child will never go hungry. The table usually holds one soup and three dishes: red rice, clam soup, sea bream, a simmered dish and pickles, each with an auspicious meaning. As in the Chinese and Korean traditions, the 100 days here include the birth day in the count.

Jewish tradition: Brit Milah on day 8

In Jewish tradition the circumcision ceremony for a boy (Brit Milah, or Bris) is held on the 8th day after birth, counted from the birth day (Jewish Virtual Library · Brit Milah). A boy born on a Sunday, for example, has his Brit Milah on the following Sunday. As with the East Asian hundred, the number 8 marks a complete cycle.

Traditional East Asian age reckoning

Separate from milestone counting, parts of East Asia historically reckoned age itself differently:

• Starts at 1 year old at birth.
• Increases by 1 at each Lunar New Year (the first day of the first lunar month), not on the birthday and not at the Gregorian new year.
• Because the count follows the lunar year, a baby born the day before Lunar New Year is already 1 year old at birth and turns 2 the very next day, only hours old.

Traditional age counts years lived through rather than time survived (Columbia University · Asia for Educators). South Korea long ran a similar system in parallel with international age, and the coexistence caused enough real confusion that the country switched to international age for legal documents on June 28, 2023 (man-nai, BBC · South Korea scraps traditional age counting).

For contrast: the everyday international system, chronological age, starts at 0 and adds a year on each birthday. It is what passports, school enrollment, legal adulthood and medical records use, and it is the “age” this article means everywhere else.

Elders chatting, fortune-telling and some zodiac-year customs may still use traditional age. For medical visits, school enrollment and legal documents, use chronological age.

Why traditions count from day 1

Medical records need to know how much time has passed, so they start at 0. Celebrations mark which day of life this is, so they start at 1. Once that distinction is clear, the same event carrying two numbers stops being confusing.

How to calculate day age and month age yourself

Any calendar app can do the subtraction: the birth day is 0, the next day is 1, and so on. MilkBaby Calendar calculates it automatically and keeps the whole family on one timeline, which matters when several caregivers share the record keeping (see how to share a baby tracker with your partner).

Month age: the same date next month

• The baby turns 1 month old on the same date in the following month. Born May 20, 1 month old on June 20, 2 months old on July 20.
• Month-end edge case: if the following month has no matching date, use its last day. A baby born on January 31 turns 1 month old on February 28 in a common year, or February 29 in a leap year.

Worked examples: one month, 100 days, first birthday

Birth date	One month (medical month age)	Traditional 100th day	First birthday
2026-03-01	2026-04-01	2026-06-08	2027-03-01
2026-01-15	2026-02-15	2026-04-24	2027-01-15
2026-01-31	2026-02-28 (common year)	2026-05-10	2027-01-31
2024-02-29 (leap year)	2024-03-29	2024-06-07	2025-02-28

The 100th-day column counts the birth day as day 1; the one-month column uses month age (same date next month, moved to the month end if needed).

References

The medical sections draw on the public references below, and the sections on customs draw on the corresponding cultural sources. Practices vary by family and region, so follow local custom.

1. WHO child growth standards
2. CDC growth charts using WHO standards
3. HealthyChildren.org (AAP) · Corrected Age for Preemies
4. AAFP · Hyperbilirubinemia in Newborns: Updated Guidelines From the AAP
5. UN Statistics Division · Notes on age statistics in population censuses
6. Columbia University · Asia for Educators: traditional Chinese age counting
7. BBC · South Korea scraps traditional age counting
8. Asia Society Korea · Baek-il: Korea’s 100-day celebration
9. Hare-kura · Okuizome guide
10. Jewish Virtual Library · Brit Milah