Facts & lineage · year boundary
The date is part of the method
A calculator can call the same early-February birth Rat, Ox or Tiger depending on which new year it silently chose. Bodhastra names the convention before printing the animal.
Three boundaries
| boundary | what it belongs to | used here? |
|---|---|---|
| 1 January | Gregorian civil year | no |
| first day, first lunar month | public lunar-calendar year and festival animal | yes |
| Li Chun | solar-term boundary used by many Four Pillars schools | not in this calculator |
The civil-date contract
The Hong Kong Observatory publishes a Gregorian-to-lunar conversion table for each year. Bodhastra stores the first day of the first month from those tables for 1901–2030 and compares the birth date as entered.
This is deliberately a civil-date calculation, not a claim that Chinese New Year begins at one simultaneous instant worldwide. A full Four Pillars calculator would need a separate location, clock and solar-term contract.
The published uncertainty
The Observatory notes that far-future new moons or solar terms very close to midnight can move a civil date by one day as astronomical estimates improve. None of its specifically flagged lunar-month cases falls inside this calculator’s 1901–2030 range.