100 Day Calculator (Baby's Hundred Days)

Dates from birth through age 18

Enter a baby's Gregorian date of birth to see eight calendar milestones: one month, 100 days, six months, and ages one, three, six, twelve, and eighteen. Rows show the Gregorian date, weekday, Chinese lunar-date label, and either a countdown or past status. The page also shows days elapsed since birth, and results update when the date changes.

How the 100-day date is counted

The 100-day row adds 100 calendar days to the date entered. In this elapsed-time convention, the birth date is day zero. If your family calls the birth date day one, the 100th day falls one day earlier: birth date plus 99 days. For an August 15, 2024 birth date, +100 days is November 23; counting August 15 as day one gives November 22. Confirm the convention before inviting guests.

Calendar months are not fixed-length spans

The one-month and six-month milestones advance by calendar months; they are not calculated as 30 or 180 days. A month-end date may land on the target month’s last valid day. Birthday milestones advance by calendar years, while 100 days is a fixed Gregorian-day count that includes weekends.

What the lunar label represents

The lunar date is another calendar label for each Gregorian milestone date. It is not produced by adding 100 lunar days. Chinese lunar months can have 29 or 30 days, and some years include a leap month. The Hong Kong Observatory publishes conversion tables for 1901–2100. Local almanacs or family conventions may differ, so check dates tied to a specific tradition.

Read and share the milestone you need

1. Choose the recorded birth date

   Select the baby's date of birth in the Gregorian calendar. Check the date against your record.

2. Compare the milestone rows

   Use the Gregorian date and weekday to schedule; upcoming rows show a countdown and past rows are marked past.

3. Review the counting convention

   For a 100-day event, confirm whether the family means 100 days after birth or the 100th day with birth counted as day one. The displayed calculation uses the first convention.

4. Copy a row if useful

   Copy a row’s name, Gregorian date, weekday, and lunar label for a note or message.

Questions about milestone dates

Check before relying on a date

• The calculation does not account for birth time, local day boundaries, or family-specific counting customs.
• The calendar conversion reference cited here covers 1901–2100. For other years or events tied to a local almanac, check a trusted calendar source.
• Milestone dates do not determine medical visits, screening, or vaccination timing. Follow the schedule provided by your child's clinician.
• A copied result can reveal a child’s birth date; share only what is needed.