[Firestore] Using Vitest setSystemTime() throws error

[risalfajar]

[READ] Step 1: Are you in the right place?

Yes

[REQUIRED] Step 2: Describe your environment

• Operating System version: MacOS Sequioa 15.7.2
• Firebase SDK version: 12.5.0
• Firebase Product: Firestore
• Node.js version: 22.19.0
• NPM version: 10.9.3

[REQUIRED] Step 3: Describe the problem

When using vi.setSystemTime() on integration tests, saving a Date to Firestore throws error.

Steps to reproduce:

1. Setup Admin NodeJS SDK
2. Setup Vitest & integration tests
3. Use vi.setSystemTime()
4. Save data containing a Date to Firestore
5. Error

Error: Value for argument "data" is not a valid Firestore document. Input is not a plain JavaScript object (found in field "date").

Relevant Code:

it("date test", async () => {
	vi.setSystemTime(new Date())
	admin.firestore.collection("tests").doc("1").set({ date: new Date() }) // throws error
	vi.useRealTimers()
})

If I were to log the new Date() it will return this: {"severity":"INFO","message":"MockDate 2025-11-27T14:51:53.132Z"}