Baby weeks and calendar months
“Four weeks old” and “one month old” use different clocks. One follows seven-day blocks; the other follows dates on a real calendar.
Two ways to count the same days.
Four weeks always means 28 days
Calendar months have 28, 29, 30 or 31 days. A four-week interval can match one February interval, but that coincidence does not make it a general month conversion.
For example, January 1 to January 29 is 28 days, or 4 weeks, but it is still 0 calendar months and 28 days. January 1 to February 1 is one calendar month (31 total days).
A year is not exactly 52 weeks
52 weeks equals 364 days. A non-leap calendar year has 365 days and a leap year has 366. A first birthday is calculated by moving the birth date forward 12 calendar months, not by adding 52 weeks.
This calculator uses February 28 for a February 29 baby’s anniversary in a non-leap year. That is a stated software convention, not a legal determination.
Keep each unit attached to its own definition
Use weeks for complete seven-day intervals, total days for elapsed dates, and calendar months for monthly anniversaries. You can quote more than one unit together without converting one into the other.
From 2024-01-01 to 2024-03-15, 2 months and 14 days is the same interval as 10 weeks and 4 days. There is no contradiction: the months include a 31-day January and a 29-day February.
Is 12 weeks the same as 3 months?
Twelve weeks is exactly 84 days. For a baby born on January 1, 2025, 12 weeks ends on March 26: calendar age is 2 months and 25 days. Three calendar months arrives on April 1, 90 days after birth, or 12 weeks and 6 days.
Use a week result when the question is about seven-day intervals and a calendar-month result when the question is about monthly anniversaries. Do not round one into the other.