The wheel moves quickly
The planets move at very different speeds, but the chart wheel turns quickly with Earth's rotation. Roughly four minutes of clock time corresponds to about one degree of rotation. That makes the birth time especially important for the Ascendant, Midheaven, and house cusps.
Angles and house boundaries
A small difference does not always change a rising sign, but it can move an angle enough to alter close aspects or shift a planet near a house boundary. Larger uncertainty can change the entire house structure. The chart may still contain useful planetary signs and aspects, but it should not display invented precision.
Historical time zones matter
Historical time zones add another layer. A place may have changed offsets, observed daylight saving under different rules, or changed political jurisdiction. Applying today's offset to an older birth can move the chart by an hour. Geographic coordinates and the historical IANA time-zone record should be resolved together.
Unknown time should remain unknown
When the time is unknown, the honest response is not to silently choose noon. Calculate what remains stable, omit time-dependent angles and houses, and label the limitation. When the time is approximate, preserve the range and show what changes within it.
Precision must be earned
Good astrology begins with data discipline. A chart that looks precise is not necessarily a chart that was calculated precisely.
