Wrong cookie expiration date value

Hi all,
I try to use custom script to add some cookies in each scenario but the expiration date of the cookie is wrong. She's set to 1969-12-31T23:59:59.000Z while she's set to 1560937443 in my cookie.json file.
So my cookie is ignored.

- This is my env
node v10.2.1, BackstopJS v3.2.16

- This is an extract of my backstop.json config :
`"onBeforeScript" : "chromy/onBefore.js",
"cookiePath": "backstop_data/engine_scripts/cookies.json"`

- This is my cookie.json file :
`[
{
"domain": ".XXXX.fr",
"expirationDate": 1560937443,
"hostOnly": false,
"httpOnly": false,
"name": "__55CC",
"path": "/",
"sameSite": "no_restriction",
"secure": false,
"session": false,
"storeId": "0",
"value": "{\"ccbm\":51,\"ccus\":1519737655379}",
"id": 22
}
]`

I've tried with puppeteer and chromy and there is the same result.
Have you any idea to explain why the expiration date of cookie is wrong ?
Thanks in advance.
Regards,