How to calculate your baby’s age
Start with two dates: your baby’s date of birth and the date you want to compare. You do not need a birth time, name, account or an average-month conversion.
1. Count elapsed calendar days
The birth date is day zero. From January 1 to January 1, zero days have elapsed; from January 1 to January 2, one day has elapsed. This is an elapsed-age convention, not an inclusive count of dates.
The calculator treats entries as dates, independent of time zones and daylight-saving changes. It does not subtract timestamps from two local midnights.
2. Divide days into weeks
Each week is exactly seven days. Divide elapsed days by seven: the whole-number part gives completed weeks, and the remainder gives extra days. Seven days is 1 week and 0 days; 28 days is 4 weeks and 0 days.
Keep the remainder. Rounding a result to the nearest week would erase useful information.
3. Count completed calendar months
Find the latest monthly anniversary on or before the comparison date. Count full calendar months to that anniversary, then count elapsed days from it to the comparison date.
For 2024-01-01 to 2024-03-15, the last anniversary is March 1. The result is 2 months and 14 days. The same interval is 74 days, or 10 weeks and 4 days. All three descriptions are exact.
4. Check your dates before using the result
Choose a comparison date on or after birth. A future comparison date is allowed if you are planning an anniversary; it is not a prediction of development. Use corrected-age mode explicitly if you also need an expected-due-date starting point.
How old will my baby be on a future date?
Keep the real birth date and choose a later comparison date. The same date rules apply; the calculator does not assume every result refers to today.
For a January 1, 2025 birth, a July 4, 2025 comparison gives 6 calendar months and 3 days, or 184 elapsed days. Planning a visit or celebration with a future date does not forecast a developmental skill.