function calculateExpiryDate(nowMs, epiresInS) {
    return nowMs + ((epiresInS-10)*1000)
}
Calculate the expiry date. Advance the expiry time by 10 seconds
to reduce the liklihood of working with an expired token.

@param {number} nowMs Current time milliseconds.
@param {number} epiresInS Expires in (seconds)
@returns