Google Directory API convert timestamp to DateTime

I'm getting info about chromeos devices through the directory API. https://developers.google.com/admin-sdk/directory/reference/rest/v1/chromeosdevices The autoupdateexpiration field returns an int64 value for this.
In C# when I try to convert to a datetime with new DateTime(autoupdatexpiration) I don't get a valid date. An example of a device returns this: 1843455600000 In the Google Admin console, the auto update expiration for this device is showing Jun 2028.
How do I convert this int to a DateTime with value Jun 2028