in src/main/scala/com/gu/hmac/HMACHeaders.scala [92:96]
def createHMACHeaderValuesWithSecret(secretKey: String, uri: URI): HMACHeaderValues = {
val now = new DateTime(clock.instant().toEpochMilli())
val hmacValue = HMACSignatory.sign(secretKey, now, uri)
HMACHeaderValues(date = now.toRfc7231String, token = hmacValue.toHeaderValue)
}