ACCURACY
Can you calculate BaZi without a birth time?
Yes—but the honest result is a three-pillar chart. The Hour Pillar should be excluded rather than invented from a default time.
What remains available
The Year, Month and Day Pillars, Day Master, Month Command and many visible relationships can still be calculated from a known date and birthplace.
What becomes unavailable
The Hour Pillar, its hidden stems and any conclusion that depends on that position must be withheld. Some timing details can also be less precise.
Why noon is not an answer
Software may need a technical placeholder to process the date, but a placeholder must never be displayed or interpreted as the real Hour Pillar.