One division of the Trimshamsha spans exactly 1 degree, and the zodiac holds 360 of them, one for each degree. The ascendant crosses one degree in about four minutes.
Four minutes is shorter than the rounding in most birth records, so the trimshamsha of the ascendant is rarely a stable quantity. This matters less here than it would elsewhere, because the D-30 is not usually cast and read as a house chart at all. The divisions are not signs; they are spans allotted to the five non-luminous planets, so a planet in the D-30 falls under Mars, Saturn, Jupiter, Mercury or Venus, and the reading is of that lordship rather than of a position in a wheel.
For the planets themselves the tolerance is workable. The Moon crosses one degree in about an hour and fifty minutes, and the Sun in about a day, so a chart from a time known to the hour still supports a D-30 reading of everything except the ascendant and the Moon near a boundary.
The Trimshamsha is one of only six charts present in all four classical groupings, the shadvarga, saptavarga, dashavarga and shodashavarga, which is a striking weight for a chart concerned with misfortune, affliction and moral trial. The tradition holds that a nativity is not read honestly until its liabilities have been read too.