Restore Date object support broken by PR #556

[adspacheco]

Purpose (TL;DR) - mandatory

Fix cross-realm Date regression introduced in #556.

Background (Problem in detail)

The public API accepts Date | number, but the change in #556 from typeof epoch.getTime === "function" to instanceof Date broke support for Date objects from different realms (iframes, vm contexts, etc.), since instanceof fails across realms.

Error: TypeError: now should be milliseconds since UNIX epoch

Reproduction:

const vm = require("vm");                                                                                                                 
const FakeTimers = require("@sinonjs/fake-timers");                                                                                       
const crossRealmDate = vm.runInNewContext("new Date('2026-04-02')");                                                                      
const clock = FakeTimers.install();                                                                                                       
clock.setSystemTime(crossRealmDate); // throws TypeError                                                                                  

Solution

Restore the original duck-typing approach (typeof epoch.getTime === "function") that worked before #556.

[fatso83]

added the missing regression test in aa15fde for you, as I introduced the regression 😉

[fatso83]

Unfortunately, this fix was incomplete, as the typing now breaks. This is what I get for not ensuring the types are built as part of the github actions. At least I know that now 😄

src/fake-timers-src.js:642:26 - error TS2339: Property 'getTime' does not exist on type 'number | Date'.
  Property 'getTime' does not exist on type 'number'.

642         if (typeof epoch.getTime === "function") {
                             ~~~~~~~

src/fake-timers-src.js:643:26 - error TS2339: Property 'getTime' does not exist on type 'number | Date'.
  Property 'getTime' does not exist on type 'number'.

643             return epoch.getTime();
                             ~~~~~~~


Found 2 errors in the same file, starting at: src/fake-timers-src.js:642

I'll get it fixed.

[fatso83]

types are now always built and the type regression fixed in d43b719

release 15.3.1 out in a few jiffies