diff --git a/src/TimeOfDay.test.ts b/src/TimeOfDay.test.ts
new file mode 100644
index 0000000..530d9a8
--- /dev/null
+++ b/src/TimeOfDay.test.ts
@@ -0,0 +1,54 @@
+import {dayMsToDecimal, decimalToDayMs} from './TimeOfDay';
+
+describe('dayMsToDecimal', () => {
+    it('works', () => {
+        const expected = {hour: 0, minute: 0, second: 0};
+        let ms = 0;
+
+        function hour() {
+            for (let i = 0; i < 10000; ++i) {
+                expect(dayMsToDecimal(ms++)).toStrictEqual(expected);
+                expect(dayMsToDecimal(ms)).toStrictEqual(expected);
+                ms += 862;
+                expect(dayMsToDecimal(ms++)).toStrictEqual(expected);
+
+                if (++expected.second >= 100) {
+                    expected.second = 0;
+                    if (++expected.minute >= 100) {
+                        expected.minute = 0;
+                        ++expected.hour;
+                    }
+                }
+            }
+        }
+
+        hour();
+        hour();
+        expected.hour += 7;
+        ms += 864 * 10_000 * 7;
+        hour();
+
+        expect(ms).toStrictEqual(86_400_000);
+        expect(expected).toStrictEqual({hour: 10, minute: 0, second: 0});
+    });
+});
+
+describe('decimalToDayMs', () => {
+    it('works', () => {
+        function hour(hour: number) {
+            let expected = hour * 8_640_000;
+
+            for (let minute = 0; minute < 100; ++minute) {
+                for (let second = 0; second < 100; ++second) {
+                    expect(decimalToDayMs({hour, minute, second})).toStrictEqual(expected);
+                    expected += 864;
+                }
+            }
+
+            expect(expected).toEqual((hour + 1) * 8_640_000);
+        }
+
+        hour(0);
+        hour(9);
+    });
+})
diff --git a/src/TimeOfDay.tsx b/src/TimeOfDay.tsx
index abc086e..8e66715 100644
--- a/src/TimeOfDay.tsx
+++ b/src/TimeOfDay.tsx
@@ -17,10 +17,9 @@ function currentDayMs(): number {
 }
 
 export function dayMsToDecimal(dayMs: number): DecimalTimeStamp {
-    const dayFrac = dayMs / 86_400_000;
-    const hour = Math.floor(dayFrac * 10);
-    const minute = Math.floor(dayFrac * 10 % 1 * 100);
-    const second = Math.floor(dayFrac * 1000 % 1 * 100);
+    const second = Math.floor(dayMs / 864 % 100);
+    const minute = Math.floor(dayMs / 86_400 % 100);
+    const hour = Math.floor(dayMs / 8_640_000);
     return {hour, minute, second};
 }