Last weekend two forms on the family table needed numbers from me at the same time. One was my aunt's registration for a community seniors' activity, asking how old she is in completed years as of a given day; the other was kindergarten registration for my cousin's son, asking how old he is on the cut-off date. At dinner we also argued about something else: a relative on the other side of the family was born on 2000-02-29, and in a common year, which day counts as turning 18. Every "days left" and "years and months" below is taken at the calculation time 2026-10-21; move that time by a day and the countdown and the months-and-days difference both have to be redone.
1. Completed years, nominal age and converted years are three different numbers
I start with my own row as the baseline: born 1996-03-15 (Friday), at this time I am 30 years 7 months 6 days, nominal age 31, about 30.602 years when converted by tropical years, 367 whole months and 11,177 total days. Each row answers a different question:
- 30 years 7 months 6 days: completed years only stand after the most recent birthday,
2026-03-15, and the months and days run from that day to the calculation time - Nominal age 31: year only, calculation year minus birth year plus 1
- 367 whole months, 1,596 weeks and 5 days, 268,248 hours, 16,094,880 minutes: four scales for the same stretch of days
- 8 leap years experienced: simply the number of leap years between
1996and2026
The countdown hangs on the same moment: the previous birthday 2026-03-15 was 220 days ago, when I turned 30; the next birthday 2027-03-15 is 145 days away, when I turn 31; the span between these two birthdays is 365 days, and 220 plus 145 is exactly that.
Two more rows are conversions only, not conclusions: 7,983 working days and 1,117,700,000 heartbeats, with their conventions explained in the questions below.
2. A leap-day birthday has two anniversary conventions in common years
For the person born on 2000-02-29 (Tuesday), the page offers an extra choice when a leap day is involved: treat the birthday as 28 February in common years, or as 1 March. I ran both options in the Age Calculator and lined them up at 2026-10-21:
| Same item | Common year mapped to 28 February | Common year mapped to 1 March |
|---|---|---|
| Completed years and months/days | 26 years 7 months 23 days | 26 years 7 months 20 days |
| Next birthday | 2027-02-28, 130 days away | 2027-03-01, 131 days away |
| Previous birthday | 2026-02-28, 235 days ago | 2026-03-01, 234 days ago |
| Turns 12 | 2012-02-29 | 2012-02-29 |
| Turns 18 | 2018-02-28 | 2018-03-01 |
| Turns 100 | 2100-02-28 | 2100-03-01 |
| Total days | 9,731 days | 9,731 days |
The two lines diverge only in common years: 2025, 2026 and 2027 fall on 28 February and 1 March respectively; 2028 has a 29 February, and both sides return to 2028-02-29. That is why the turning-12 cell shows the same date and the turning-18 cell splits only at 2018. The leap-year rule is that an ordinary year divisible by 4 is a leap year, and a year divisible by 100 but not by 400 is not. The page's samples: 1900 common, 2000 leap, 2024 leap, 2026 common, 2100 common, 2400 leap. 2100 being common is exactly why the turning-100 cell is one day later.
"Which day the law requires" is not mine to decide: local practice and each office's rules already differ, and the page only puts two results side by side. I copy whatever the clerk writes on the back of the form. Apply the same method to a child born 2024-02-29 (Thursday): at this time the child is 2 years 7 months 23 days old, 965 total days, 1 leap year experienced, and the next birthday 2027-02-28 is 130 days away, when the child turns 3. That 130 days is exactly what the kindergarten cut-off hangs on.
3. The zodiac follows the start of spring; the page uses the 4 February line
There are three ways to assign an animal sign: New Year's Day, Spring Festival and the start of spring. The label the page prints says zodiac (by start of spring), takes 4 February as the approximate line, and puts births before 4 February in the previous zodiac year.
| Date of birth | Zodiac given by the page |
|---|---|
2024-02-03 / 2024-02-04 | Rabbit (guimao year) / Dragon (jiachen year) |
2025-02-03 / 2025-02-04 | Dragon (jiachen year) / Snake (yisi year) |
2026-02-03 / 2026-02-04 | Snake (yisi year) / Horse (bingwu year) |
Two birth certificates one day apart make the point best: born 2025-02-03 gives 1 year 8 months 18 days, 625 total days, Aquarius, dragon (jiachen year); born 2025-02-04 gives 1 year 8 months 17 days, 624 days, snake (yisi year). One day apart, one day fewer, the animal sign changes year, the star sign does not move. The exact minute of the start of spring is beyond the page's precision; how ganzhi is derived, and why it often disagrees with the first day of the first lunar month, is covered in more detail in Ganzhi years and the zodiac.
4. What paperwork needs is the day you turn the age
My aunt was born 1949-10-01 (Saturday). At this time she is 77 years 0 months 20 days, nominal age 78, 28,144 total days, 4,020 weeks and 4 days, 19 leap years experienced, an ox (jichou year), Libra. Her previous birthday 2026-10-01 was only 20 days before the calculation time, so that row reads "77 years and 20 days". Her next birthday 2027-10-01 is 345 days away, when she turns 78; turning 80 falls on 2029-10-01, 1,076 days away. The community line is drawn at 80, so I copied that row onto the back of the registration form.
Push both conventions to higher ages: turning 70 falls on 2070-02-28 (15,836 days away) and 2070-03-01 (15,837 days), one day apart; turning 80 shows 2080-02-29 in both cells with 19,489 days to go, because that year has a 29 February and the two lines merge again; turning 100 splits again into 2100-02-28 (26,793 days) and 2100-03-01 (26,794 days). My own turning 100 is 2096-03-15, 25,348 days away.
The range is worth stating too: birth years can only be 1900 to 2100, which the page marks as necessary for consistent anniversary and zodiac conventions; the calculation time can be moved to a past date, so "how old on the day of school entry" comes out directly. Some systems store a birthday as a number of seconds; for those I switch to the Timestamp Converter to check which instant that day corresponds to. Day counts are not affected by time zones; both dates are compared as UTC calendar days.
Q: What exactly is the difference between completed years and nominal age?
Completed years only increase after the birthday actually passes; nominal age looks at the year difference alone, so the two pairs above differ by 1: 30 against 31, 77 against 78. For someone born early in the year, different starting lines can make a difference of 2. The page computes nominal age from the Gregorian year difference.
Q: Born on 29 February, which day do I turn 18?
The two common conventions are 2018-02-28 and 2018-03-01. The tool shows the results side by side and does not rule; for anything you hand to an outside office, write the convention that office accepts, and do not treat the page as a legal basis.
Q: Can I quote the working-day and heartbeat figures?
Only with their conventions attached: 7,983 working days is 11,177 days times 5 divided by 7, rounded down, with no public holidays deducted; 1,117,700,000 heartbeats assumes about 100,000 a day, which is an order of magnitude, not a physiological measurement. What is actually precise is the day count, the years, months and days, and the milestone dates.
Q: Is the zodiac counted from Spring Festival or from the start of spring?
This page uses the start of spring, with 4 February as the approximate line, so neither New Year's Day nor the first day of the first lunar month. Folk custom mostly uses the first day of the first lunar month; fortune-telling and the ganzhi calendar mostly use the start of spring. When both conventions exist, saying which one you used is enough.
Back to "which day do I actually turn 18". My choice is to give both, writing 2018-02-28 and 2018-03-01 into the note. 2018 is a common year and 29 February does not exist, so the day never had a single answer; for whichever office you deal with, write the convention on their form, and on other occasions use the day the birthday person prefers to celebrate. What must be pinned down is the calculation time: counting forward from 2026-10-21 gives 130 or 131 days, two dates rather than one conclusion, and it changes by itself once that time passes.