True solar time
Clock time corrected to where the sun actually stood at your birthplace.
In one sentence
Clock time corrected to where the sun actually stood at your birthplace.
How it is used
A country keeps one standard time across a wide span of longitude, and clocks also move for daylight saving, so the hour on a birth record can sit a long way from local noon. Since the hour pillar changes on a two-hour boundary, this correction decides which hour pillar you end up with. It is arithmetic on time zones, longitude and the equation of time, not a mystical adjustment.