Auth0 Jwt strange behavior - jwt

Question

I want to use jwt as authentication mechanism so I pick auth0-jwt java libs. To implement authentication services I did a test about token validation. The test is simple, use this token:
{ "alg":"HS256", "typ": "JWT" }
{ "sub": "dummySubject", "exp": "1498054620653" }
with supersecretpassphrase to verify signature. Given that token I expect back this (taken by jwt.io) one:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJkdW1teVN1YmplY3QiLCJleHAiOiIxNDk4MDU0NjIwNjUzIn0.VTL9gIQ-POIoI8W0OI9y_Q7rDGFbniUGNcfd_EliK60
but test fails. The returned token is:
eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJkdW1teVN1YmplY3QiLCJleHAiOjE0OTgwNTQ2MjB9.2dOy3HmWH7vMWiJoQvMOUsx2nR6l6phZ1nwIz-TSkNk
jwt.io says that that token has this shape:
{ "typ": "JWT", "alg": "HS256" }
{ "sub": "dummySubject", "exp": "1498054620" }
It looks like exp loose its milliseconds. This is how I fill my token:
private String computeToken(JwtToken token) throws Exception {
Algorithm algorithmHS = Algorithm.HMAC256(passphrase);
Date expirationTime = new Date(token.getExpirationTime().toInstant().toEpochMilli());
Builder jwtToken = JWT.create().withSubject(token.getSubject()).withExpiresAt(expirationTime);
Iterator<Entry<String, Object>> candidateClaimsIterator = token.getPayload().entrySet().iterator();
...
return jwtToken.sign(algorithmHS);
}
The JwtToken token parameter is a simple bean I use to ship and validate data I want to put in token,
public class JwtToken implements Serializable {
private static final long serialVersionUID = 1L;
#NotNull(message = "Subject can not be null")
#Size(min = 10)
private String subject;
#NotNull(message = "Expiration time can not be null")
private ZonedDateTime expirationTime;
private Map<String, Object> payload;
...
(getters and setters)
}
It seems that java.util.Date loose it's milliseconds while com.auth0.jwt.JWTCreator convert exp in a JSON string. Am I doing something wrong? Is this the expected behavior?