The Chinese day is divided into twelve watches of two hours each, named for the twelve earthly branches: 子 begins at 11pm, 午 holds noon. Your birth falls inside one of them, and that watch — paired with a heavenly stem — becomes your hour pillar.
In classical reading, the year pillar speaks of ancestry and context, the month of upbringing and season, the day of the self. The hour pillar is the most intimate: it is read as your inner life, your later years, and the part of you that emerges when nobody is watching.
This is why serious readings insist on the hour, and why Kairos asks for it gently but hopefully. A chart without it can still be read — three pillars carry six of the eight characters — but the reading loses its most personal register.
If you don't know your hour, ask family, or check a birth record. And if it is truly lost, know this: the tradition read three-pillar charts for centuries. The pattern is dimmer, not gone.
The 子 watch straddles midnight, and schools differ. Kairos follows the convention that the watch belongs to the day in progress; a full written reading will note both possibilities.
Yes — the watches follow the sun, not the clock. Kairos adjusts for your birthplace when computing the hour pillar.