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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.
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