The square itself
The arrangement is fixed: 4-9-2 across the top, 3-5-7 across the middle, 8-1-6 across the bottom. Every line through it — three rows, three columns, two diagonals — adds to fifteen.
In China it is the pattern said to have been found on a turtle's shell in the river Lo. In India the same square is the Saturn yantra, and the numbers are read as planets: 1 is the Sun, 2 the Moon, 3 Jupiter, 4 Rahu, 5 Mercury, 6 Venus, 7 Ketu, 8 Saturn, 9 Mars.
Plotting a birth date
Write the date as DD-MM-YYYY with leading zeros and place each digit in its own cell. 05-09-1994 gives 5, 9, 1, 9, 9 and 4 — the two zeros are dropped, because the square has no cell for zero.
A digit that appears three times is written three times in its cell and read as amplified. A digit that never appears is a missing number.
Planes
Each line through the square is a plane, and a plane whose three numbers are all present is read as a completed strength. The rows are the mental (4-9-2), emotional (3-5-7) and practical (8-1-6) planes; the columns are thought (4-3-8), will (9-5-1) and action (2-7-6).
The two diagonals are singled out: 4-5-6 is the Golden Yoga and 2-5-8 the Raja Yoga. Both pass through the centre, which is why a missing 5 is read as the most structurally significant gap — without a centre, the other numbers have nothing to organise around.
Why 21st-century dates look sparse
Every date from 2000 to 2009 contains two zeros that go nowhere, and dates in this century generally carry fewer distinct digits than dates in the last. Grids for children born now are visibly emptier than their parents', and that is an artefact of the calendar rather than a statement about a generation.
It is worth saying plainly because the alternative reading — that a whole cohort is deficient — gets sold, and it is nonsense.