The Sun's apparent ecliptic longitude is divided into 24 equal segments of 15°. Each crossing is a 'solar term' — observable to ancient astronomers via shadow length and equinox/solstice timing.

In BaZi/Saju, 12 of the 24 solar terms mark month-pillar boundaries:

• Yin month (寅) starts at Ipchun (立春, sun at 315°, ~Feb 4)
• Mao month (卯) starts at Jingzhe (驚蟄, sun at 345°, ~Mar 6)
• ... and so on, completing the lunar months

Two people born 2 days apart in early February can have entirely different month pillars if the boundary fell between them.

My reader computes all 24 to the minute: https://tarofortune.pythonanywhere.com/en