diff --git a/apps/web/src/components/timeline/dragMath.test.ts b/apps/web/src/components/timeline/dragMath.test.ts
new file mode 100644
index 0000000..e8fa77b
--- /dev/null
+++ b/apps/web/src/components/timeline/dragMath.test.ts
@@ -0,0 +1,43 @@
+import { describe, expect, it } from "vitest";
+import { computeDragDates, pixelsToDays, shiftDate } from "./dragMath.js";
+
+describe("dragMath", () => {
+  it("returns zero days for invalid cell widths instead of leaking invalid drag deltas", () => {
+    expect(pixelsToDays(40, 0)).toBe(0);
+    expect(pixelsToDays(40, Number.NaN)).toBe(0);
+    expect(pixelsToDays(40, Number.POSITIVE_INFINITY)).toBe(0);
+  });
+
+  it("shifts dates without mutating the original date", () => {
+    const original = new Date("2026-04-10T00:00:00.000Z");
+
+    const shifted = shiftDate(original, 3);
+
+    expect(shifted.toISOString()).toBe("2026-04-13T00:00:00.000Z");
+    expect(original.toISOString()).toBe("2026-04-10T00:00:00.000Z");
+  });
+
+  it("clamps resize-start drags so the start does not cross the end", () => {
+    const { start, end } = computeDragDates(
+      "resize-start",
+      new Date("2026-04-10T00:00:00.000Z"),
+      new Date("2026-04-12T00:00:00.000Z"),
+      5,
+    );
+
+    expect(start.toISOString()).toBe("2026-04-12T00:00:00.000Z");
+    expect(end.toISOString()).toBe("2026-04-12T00:00:00.000Z");
+  });
+
+  it("clamps resize-end drags so the end does not cross the start", () => {
+    const { start, end } = computeDragDates(
+      "resize-end",
+      new Date("2026-04-10T00:00:00.000Z"),
+      new Date("2026-04-12T00:00:00.000Z"),
+      -5,
+    );
+
+    expect(start.toISOString()).toBe("2026-04-10T00:00:00.000Z");
+    expect(end.toISOString()).toBe("2026-04-10T00:00:00.000Z");
+  });
+});
diff --git a/apps/web/src/components/timeline/dragMath.ts b/apps/web/src/components/timeline/dragMath.ts
index 9735c08..0bbd6eb 100644
--- a/apps/web/src/components/timeline/dragMath.ts
+++ b/apps/web/src/components/timeline/dragMath.ts
@@ -6,6 +6,9 @@
 
 /** Convert a pixel delta to a number of whole days based on cell width. */
 export function pixelsToDays(deltaX: number, cellWidth: number): number {
+  if (!Number.isFinite(cellWidth) || cellWidth <= 0) {
+    return 0;
+  }
   return Math.round(deltaX / cellWidth);
 }