--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ b/internal/ledger/rounding.go	2026-03-04 11:02:19.000000000 +0000
@@ -0,0 +1,17 @@
+package ledger
+
+// HalfEven rounds a minor-unit amount to the nearest multiple of step using
+// banker's rounding, which keeps long runs of postings from drifting upward.
+func HalfEven(minor, step int64) int64 {
+	if step <= 1 {
+		return minor
+	}
+	q, r := minor/step, minor%step
+	switch {
+	case 2*r > step:
+		q++
+	case 2*r == step && q%2 != 0:
+		q++
+	}
+	return q * step
+}