Across a leap-year February
January 1 → March 15, 2024
74 days = 10 weeks and 4 days. January 1 to March 1 is two completed calendar months, followed by 14 days. Calendar age: 2 months and 14 days.
A little clarity, every day
Baby Age in Days
Exact days since birth, starting from day zero. Weeks, months and your 100-day date, together.
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Your baby's dates
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Your result
Every little day counts.
Add a birth date to see exact calendar months, weeks and days, plus the next dates worth remembering.
monthsweeksdays
Count the days that have passed, starting at zero.
The birth date is age 0 days; the following date is age 1 day. We compare calendar dates rather than birth times, so a daylight-saving change never adds or removes an age day. Total days count time already elapsed, while a cultural “100th day” that includes birth as day 1 uses a different convention.
28days in four weeks
≠4 × 7. Always.
28–31days in a calendar month
The month and year decide.
Four weeks can line up with one calendar month in a particular example, but it is not a conversion rule. Fifty-two weeks is 364 days, so it is not exactly twelve calendar months either.
Why weeks and months do not matchFor newborns, infants and toddlers
The same date engine works from the day of birth onward. Choose the unit that makes the date easiest to read; the display is not a medical assessment.
- Starting at day zero
- For a newborn, elapsed days and completed weeks can be easiest to follow. The same birth and assessment date always gives age 0 days.
- Keeping the calendar in view
- For infants, calendar months plus leftover days keep monthly anniversaries tied to the original birth date. Do not divide weeks by four to obtain calendar months.
- As your baby grows
- For toddlers, the same calendar-month calculation continues alongside weeks and total days. It counts elapsed time without judging development or predicting a skill.
Check the dates yourself
Reproducible examples, with the counting rules visible.
A birth at the end of January
January 31 → February 28 → March 31, 2025
At February 28: 1 month, 0 days. At March 31: 2 months, 0 days. Each anniversary starts from January 31, so the short February does not move March’s anniversary to the 28th.
Born at 32 weeks
Birth January 1 → assessment April 23, 2025
112 elapsed days = 16 weeks. The 40-week reference is 56 days after birth, on February 26. Corrected age on April 23 is 56 days = 8 weeks. These weeks are not converted into calendar months.
Celebrate the date. Leave room for their own pace.
A total-day count makes date anniversaries easy to reproduce. Age 100 days is birth plus 100 days; monthly anniversaries still follow the calendar.
A date milestone marks time. It does not say when a baby should walk, talk, or reach another skill.
Chronological and corrected age, explained
Use the visible corrected-age mode for a baby born early. Enter either the original expected due date or completed gestational weeks plus 0–6 extra days.
Chronological age
Elapsed time from the birth date to the assessment date. It stays visible in both modes.
Corrected (adjusted) age
Elapsed time from the expected due date to the assessment date. Before that date, the tool shows a countdown.
Corrected days = elapsed days − (280 − gestational days at birth)
The formula uses 40 weeks (280 days) as a calculation reference. That reference does not define the only medical meaning of “full term.” Ask your baby’s clinician how corrected age applies to their care.
Read the chronological vs. corrected age guideDevelopment has its own guidance
Keep developmental checklists separate from celebration dates. These external resources explain their own age categories and limitations.
CDC: developmental milestone checklists
CDC publishes infant checklists at 2, 4, 6, 9, and 12 months—not for every month. Visit the source to choose an available checklist.
American Academy of Pediatrics: corrected age
Parent guidance about using corrected age in the first two years. Your baby’s clinician can explain its use in their individual care.
This calculator cannot screen for developmental delay. If you have concerns, discuss them with your baby’s clinician; a date calculation cannot assess a skill or predict its arrival.
A few things parents ask
Is my baby one month old at four weeks?
Not as a general rule. Four weeks is 28 days. A calendar month is 28–31 days, so the exact monthly anniversary depends on the birth date, the target month, and leap years.
What happens if there is no 29th, 30th, or 31st that month?
We use the last valid day of that month, then calculate later anniversaries from the original birth date again. January 31 becomes February 28 in 2025, then March 31. The February adjustment does not carry forward.
Does the birth date count as day 0 or day 1?
For elapsed age, it is day 0. The same birth and assessment date gives 0 days; seven calendar dates later gives 1 week and 0 days. We do not use inclusive counting.
Is corrected age the same as chronological age?
They have different start dates: birth for chronological age, expected due date for corrected age. The tool displays both. The corrected result is information to discuss with your clinician, not a developmental assessment.
Where does my baby’s birth date go?
The calculator works in your browser. It does not put entered dates into a URL or send them to a calculation service. Copying or printing creates a copy you control. Saving on this device is optional.
Can I change a date or remove saved information?
Yes. Edit the inputs and calculate again. Saving is only on this browser after you choose it. Use the clear-saved-data control to remove that saved copy; it does not delete something you already copied or printed.
Does a milestone date tell me when my baby should walk or talk?
No. This calendar marks elapsed age and date anniversaries. It cannot diagnose delay, assign a developmental score, or predict the date of a particular skill. Use appropriate clinical guidance for developmental questions.