How this calculator works
Days old is the exact number of calendar days between the birth date and the measurement date. Completed weeks divide that count by seven and drop the remainder, so a baby 92 days old is 13 completed weeks and 1 day. Completed months use the same day-of-month rule as ages in years, with a birth on the 31st clamped to the last day of a shorter month, so monthly birthdays never skip a month.
Formula
days = O(as_of) - O(birth_date), where O is the day number of a date. weeks = floor(days / 7). months is the largest whole number m for which birth_date plus m months is not after as_of. The next monthly birthday is birth_date plus m + 1 months, clamped to the month length.
Worked example
A baby born on 2026-08-01 measured on 2026-11-01 is 92 days old, 13 completed weeks old and 3 completed months old. The next monthly birthday is 2026-12-01, 30 days later.
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Frequently asked questions
Should I use weeks or months for a baby?
Health visits often use weeks until about three months, then months. Both are shown so you can match the convention your clinic uses. Weeks are exact; months follow the calendar and vary in length.
How is the monthly birthday counted?
It is the same day number each month. A baby born on January 31 has a monthly birthday on February 28 in a non-leap year, because February has no 31st, and then on March 31.
Does the time of birth matter?
No. The page works in whole calendar dates with no clock time or time zone, so the answer is the same whether the birth was in the morning or late at night.
What about a premature baby?
This is chronological age from the birth date. Corrected or adjusted age for a premature baby is measured from the due date instead, which a clinician will calculate separately.