Twelve month boundaries within twenty-four solar terms
The usual BaZi month convention changes at the twelve jie. These are the opening terms of the solar months, not all twenty-four solar terms. Li Chun (Start of Spring) opens the Yin (Tiger branch) or Tiger month; Jing Zhe (Awakening of Insects) opens the Mao (Rabbit branch) or Rabbit month; Qing Ming (Pure Brightness) opens the Chen (Dragon branch) or Dragon month. The intervening terms, such as Yu Shui (Rain Water) and Chun Fen (Spring Equinox), do not start another month under this convention. Neither the first day of a Gregorian month nor a lunar new moon substitutes for these boundaries. The month stem also depends on the year stem, so a month animal alone is not a complete Month Pillar.
Compare instants, not a date printed on a calendar
Suppose two births occur on the same local date, one before and one after the verified Jing Zhe instant. Under the jie convention, the first remains in the Tiger month. The second enters the Rabbit month, even though both birth records show the same Gregorian month. This is a hypothetical comparison, not an assertion of a particular year's transition time. Obtain the term's timestamp from a reliable ephemeris and interpret its stated timezone. Then compare it with the birth record using the birthplace's historical civil-time rules. A calendar giving only the day cannot settle a birth close to the transition.
Record what changes and what remains uncertain
When checking two calculators, keep the birth inputs identical and record the month-boundary convention, timezone conversion and solar-time setting for each. If the birth-time uncertainty spans a jie transition, retain both possible Month Pillars rather than choosing the one whose description sounds familiar. A changed Month Pillar alters the traditional seasonal interpretation; it does not by itself change the Day Master (the element of your day pillar), which is the Day Pillar's stem. Correct calendar arithmetic is checkable, while claims about personality or future events remain cultural interpretation, not scientifically established prediction.