Jest: Add setSystemTime and getRealSystemTime fn types

types/jest/index.d.ts

@@ -131,6 +131,26 @@ declare namespace jest {
      * Returns the number of fake timers still left to run.
      */
     function getTimerCount(): number;
+    /**
+     * Set the current system time used by fake timers. Simulates a user
+     * changing the system clock while your program is running. It affects the
+     * current time but it does not in itself cause e.g. timers to fire; they
+     * will fire exactly as they would have done without the call to
+     * jest.setSystemTime().
+     *
+     * > Note: This function is only available when using modern fake timers
+     * > implementation
+     */
+    function setSystemTime(now?: number): void;
+    /**
+     * When mocking time, Date.now() will also be mocked. If you for some
+     * reason need access to the real current time, you can invoke this
+     * function.
+     *
+     * > Note: This function is only available when using modern fake timers
+     * > implementation
+     */
+    function getRealSystemTime(): number;
     /**
      * Indicates that the module system should never return a mocked version
      * of the specified module, including all of the specificied module's dependencies.

types/jest/jest-tests.ts

@@ -291,6 +291,17 @@ jest.useFakeTimers('legacy');
 // $ExpectError
 jest.useFakeTimers('foo');
 
+// https://jestjs.io/docs/en/jest-object#jestsetsystemtimenow-number--date
+jest.setSystemTime();
+jest.setSystemTime(0);
+// $ExpectError
+jest.setSystemTime('foo');
+
+// https://jestjs.io/docs/en/jest-object#jestgetrealsystemtime
+const realSystemTime1: number = jest.getRealSystemTime();
+// $ExpectError
+const realSystemTime2: number = jest.getRealSystemTime('foo');
+
 // https://jestjs.io/docs/en/jest-object#jestrequireactualmodulename
 // $ExpectType any
 jest.requireActual('./thisReturnsTheActualModule');