A birth certificate records clock time. The double-hour of birth is something else. This note works through the difference between true solar time and mean solar time, and the three corrections that lie between a recorded clock time and the double-hour it must be read against. The first is daylight saving time: China observed it for six years, 1986 to 1991, and a time recorded in that window must be put back an hour. The second is longitude: Beijing time is the mean solar time of 120°E, not the local time of Beijing, and since China spans some sixty-two degrees of longitude under a single standard line, the correction reaches more than two hours in the west; the rule is one degree to four minutes, applied as 4 minutes × (local longitude − 120°). The third is the equation of time, at most about sixteen minutes fast and fourteen minutes slow, which matters only when a birth time sits close to a double-hour boundary; two of its causes are the elliptical orbit and the obliquity of the ecliptic, and it passes through zero four times a year. The note then sets out the long-standing dispute over whether the 子 hour, which spans 23:00 to 01:00 and two calendar days, should turn the day at its initial (子初) or at its midpoint (子正), citing the Gexiang xinshu and the Sanming tonghui, and declines to choose between them. It closes with a four-step checklist: establish the place of birth, check daylight saving, correct for longitude, and apply the equation of time only on a boundary. The note is descriptive; it contains no predictive claims.(This preprint has not been peer reviewed.)