What the 12-month line means
The flag on this page is a single threshold: fewer than 12 completed months between the two birth dates. Twelve months and zero days is not under the line. Eleven months and 28 days is. The calculator uses the same borrowing method as the rest of ChronoCalc, so a gap that crosses February 29 is still exact. It does not treat every month as 30 days and it does not call two children “twins” in a medical sense.
Twins share a pregnancy. Siblings born months apart do not. The slang term is only a search phrase for close spacing. Some families use it affectionately and some find it stereotypical. The result text on this page says “close spacing” or “12 months or more” so you can ignore the slang after you have the number. The age difference calculator reports the same span with no label at all.
How to read the three outputs
The prose gap is years, months, and days. The Y;M;D line is the same span with semicolons, which is the shape many score sheets use for a child’s age — here it is the gap between siblings, not either child’s age today. Total days is the check. If the prose and the day count feel inconsistent, trust the day count and re-read the months: leftover days are not a full extra month.
Order does not matter for the flag. Enter the older date first if you want the labels to match the form, but the absolute gap is the same if you swap them. Two identical dates are rejected because a gap of zero is not a spacing question. If the children were born on the same day they are twins or same-day siblings, and this threshold does not add information.
School years and the almost-12-month case
A gap of 11 months can put siblings in the same school grade or in adjacent grades depending on where their birthdays sit relative to a district cutoff. A gap of 13 months can do the same. The flag does not know your school calendar. Take both birth dates to the school age cutoff page if the question is kindergarten eligibility, not the slang label.
Close spacing is also not a medical risk score. Pregnancy interval is a clinical topic with its own literature; this page does not estimate risk, fertility, or developmental outcomes. It answers a calendar question. For each child’s age today, open the chronological age calculator once per birth date.
Both dates stay in the browser. Nothing is saved on a ChronoCalc account, because there is no account.