remove floating point rounding error in Timestamp.fromMillis() (#4745)

Author: Brian Chen

.changeset/rich-olives-tease.md

@@ -0,0 +1,5 @@
+---
+'@firebase/firestore': patch
+---
+
+Fixed a bug where decimal inputs to `Timestamp.fromMillis()` calculated incorrectly due to floating point precision loss

packages/firestore/src/lite/timestamp.ts

@@ -21,6 +21,9 @@ import { primitiveComparator } from '../util/misc';
 // The earliest date supported by Firestore timestamps (0001-01-01T00:00:00Z).
 const MIN_SECONDS = -62135596800;
 
+// Number of nanoseconds in a millisecond.
+const MS_TO_NANOS = 1e6;
+
 /**
  * A `Timestamp` represents a point in time independent of any time zone or
  * calendar, represented as seconds and fractions of seconds at nanosecond
@@ -66,7 +69,7 @@ export class Timestamp {
    */
   static fromMillis(milliseconds: number): Timestamp {
     const seconds = Math.floor(milliseconds / 1000);
-    const nanos = (milliseconds - seconds * 1000) * 1e6;
+    const nanos = Math.floor((milliseconds - seconds * 1000) * MS_TO_NANOS);
     return new Timestamp(seconds, nanos);
   }
 
@@ -138,7 +141,7 @@ export class Timestamp {
    *     the number of milliseconds since Unix epoch 1970-01-01T00:00:00Z.
    */
   toMillis(): number {
-    return this.seconds * 1000 + this.nanoseconds / 1e6;
+    return this.seconds * 1000 + this.nanoseconds / MS_TO_NANOS;
   }
 
   _compareTo(other: Timestamp): number {

packages/firestore/test/unit/api/timestamp.test.ts

@@ -134,6 +134,12 @@ describe('Timestamp', () => {
     expect(t1 >= t2).to.be.false;
   });
 
+  it('handles decimal inputs in fromMillis()', () => {
+    const actual = Timestamp.fromMillis(1000.1);
+    const expected = new Timestamp(1, 100000);
+    expect(actual.isEqual(expected)).to.be.true;
+  });
+
   it('serializes to JSON', () => {
     expect(new Timestamp(123, 456).toJSON()).to.deep.equal({
       seconds: 123,