Our calculation method.
One date engine powers all our tools. This page documents its rules so that a result can be checked with a calendar.
Same birth date. Every month.
Inputs and validation
Enter dates as YYYY-MM-DD, using the Gregorian calendar. The interface accepts dates from 1900 through 2099. Impossible dates, incomplete dates and comparison dates before birth are rejected. A future comparison date is allowed for planning; a future birth date is not used for a baby who has already been born.
No birth time is requested. Calendar-day age is the intended output, rather than an exact duration in hours.
Days and weeks
Birth is day zero. Total days are the difference between the two calendar dates, represented as UTC whole-day numbers to avoid daylight-saving offsets. Full weeks are the total divided by seven, rounded down; extra days are the remainder.
The Today shortcut reads the current date in the visitor’s device time zone. After that selection, calculations depend only on the dates entered.
Months, birthdays and anniversaries
Each monthly anniversary is computed directly from the original birth date. Keep its day number when possible; if the target month is shorter, use that month’s final day. Do not carry that shorter day forward into later months.
Completed months are the greatest number of anchored monthly anniversaries reached by the comparison date. Remaining days run from that anniversary to the comparison date. The first birthday is birth plus 12 calendar months. The 100-day date is birth plus 100 elapsed days.
Corrected-age mode
With an expected due date, count from that date while also retaining birth-date age. With gestational age, compute the difference from a 280-day reference and add it to birth to derive the due date. Gestational extra days must be integers from 0 through 6.
Forty weeks is the arithmetic reference, not the only medical definition of full term. Before the reference due date we show a countdown and no negative age.
Verification and limitations
The automated examples include day zero, 7 days, 28 days, January 31 month ends, February 29, the 74-day leap-year example, and 32 weeks plus additional days.
These are transparent software rules, not a medical assessment or legal age service. Dates alone cannot establish developmental delay or predict when a baby will gain a skill. Contact the child’s healthcare professional about individual concerns.
Why can another calculator give a different month count?
First compare conventions. Some tools divide total days by 30 or 30.4375, count birth as day one, or advance each anniversary from the previous shortened month. Those methods answer different counting questions.
Our result uses elapsed day zero and monthly anniversaries anchored to the original birth date. A meaningful comparison must use the same birth date, comparison date and month-end rule; a different number alone does not identify which rule was used.