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Baby age & milestones

Count days, weeks, completed months, and milestones.

See examples and explanations ↓

Birth is zero elapsed days. Milestones count birth as day one. When a month has no matching birth-date day, we use its last day for the anniversary.

One baby

Stores one baby’s birth date, reference date, and gestational weeks/days, together with any latest measurement inputs saved in the growth tool. Deleting here removes both. Saving a different birth date clears the previous baby’s measurement inputs. Expires 90 days after last use. Other people on a shared device may see these inputs.

Time together

Enter dates, then select Calculate.

What this tool explores

Count the time from birth to a reference date in days, weeks and completed calendar months, and find milestones that count birth as day one. See why different date conventions can produce different numbers.

Step by step

  1. Choose the birth date and the date you want to check, from birth through the 24-month anniversary.
  2. Calculate, then distinguish elapsed days from the day-one count that includes the birth date.
  3. Find the dates in the milestone table. To reuse inputs on a later visit, explicitly select Remember on this device and calculate.

What to look for in these results

Elapsed days count calendar-date differences; weeks divide that count into groups of seven and remaining days. Completed months follow calendar anniversaries, not fixed 30-day or four-week blocks. Celebratory milestones do not assess growth or development.

Questions about this tool

Why is the 100-day date one day different in another app?

Check whether birth is counted as day zero or day one. ToffeeLab starts elapsed days at zero and the 50th, 100th, 200th and 300th day milestones at one.

Is a January 31 birth two months old on March 30?

Not under this tool’s convention. In 2024, one month completes on February 29 and two months on March 31. March 30 displays one month and 30 days; passing through a shorter month does not move later anniversaries to the 29th.

Saving inputs on this device

Select Remember on this device and calculate to save inputs. On your next visit, only inputs are restored; results wait for the Calculate button. Saving is off by default. Other people using a shared device may see saved inputs.

Date conventions

Elapsed days use calendar-date differences, with birth as day zero, independent of 23- or 25-hour daylight-saving days. Completed months advance on the matching day, or the last day when a month is shorter. For example, January 31 to February 29, 2024 is 29 elapsed days and one completed month.

The 50th, 100th, 200th, and 300th day milestones count birth as day one. The 100th day is one day before 100 full days have elapsed. Input dates range from 1900 through 2100 and the reference date must fall from birth through the exact 24-month anniversary. Out-of-range inputs are rejected.

Frequently asked questions

Do I need a birth time or time zone?

This tool counts the difference between two selected calendar dates, not the exact elapsed hours. It does not request a birth time or timezone. Check the reference date yourself.

Can I calculate if storage fails?

Yes. Calculations work in memory. Device storage is optional, and a message explains when it is unavailable.

baby-dates-1.0.0 · Calculation review date: 2026-10-05

Understand the method before interpreting the result.

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