Civil, mean solar and apparent solar time are different
Start with the recorded civil time and the historical UTC offset, including daylight saving where applicable. Local mean solar time then reflects longitude: Earth rotates 360 degrees in roughly 24 hours, giving four minutes per degree. Apparent or true solar time adds a date-dependent equation-of-time correction because the apparent Sun does not run uniformly against a clock. A longitude correction alone is therefore not the complete true-solar-time calculation. BaZi schools differ over which time convention to use; a calculator should name its method rather than imply all schools require the same correction.
A worked longitude example near an hour boundary
As a hypothetical example, take a place five degrees west of its standard-time reference meridian, with no daylight saving. The longitude-only correction puts local mean solar time twenty minutes behind standard civil time. A recorded 09:10 would become 08:50 before the equation-of-time adjustment. That crosses the conventional 09:00 division between Chen (Dragon branch) and Si (Snake branch) hours, showing why the method can matter. It does not establish the final apparent solar time or the hour stem: those still require the date-dependent correction and the relevant Day Stem. Do not apply this example as a universal twenty-minute subtraction.
Preserve the record and compare declared methods
Keep the original birth certificate time, birthplace, date, historical offset and each correction in separate fields. If the calculator already handles daylight saving or solar time, manually applying it again introduces a second correction. Compare the resulting pillars under the methods being considered and mark which conclusions depend on the uncertain hour. Near a day boundary, also record whether the school changes the day at midnight or uses a Zi (Rat branch)-hour convention. Do not choose a method because a personality description feels right; agreement with a story cannot verify a timestamp or establish predictive accuracy.