Bun's test runner supports setting the system time programmatically with the setSystemTime function.
import { test, expect, setSystemTime } from "bun:test";
test("party like it's 1999", () => {
const date = new Date("1999-01-01T00:00:00.000Z");
setSystemTime(date); // it's now January 1, 1999
const now = new Date();
expect(now.getFullYear()).toBe(1999);
expect(now.getMonth()).toBe(0);
expect(now.getDate()).toBe(1);
});
The setSystemTime function is commonly used on conjunction with Lifecycle Hooks to configure a testing environment with a deterministic "fake clock".
import { test, expect, beforeAll, setSystemTime } from "bun:test";
beforeAll(() => {
const date = new Date("1999-01-01T00:00:00.000Z");
setSystemTime(date); // it's now January 1, 1999
});
// tests...
To reset the system clock to the actual time, call setSystemTime with no arguments.
import { test, expect, beforeAll, setSystemTime } from "bun:test";
setSystemTime(); // reset to actual time
See Docs > Test Runner > Date and time for complete documentation on mocking with the Bun test runner.