diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/Authorization.java b/acme4j-client/src/main/java/org/shredzone/acme4j/Authorization.java
index 69d155e7..510f51dc 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/Authorization.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/Authorization.java
@@ -16,7 +16,6 @@ package org.shredzone.acme4j;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
-import java.util.Date;
 import java.util.List;
 
 import org.shredzone.acme4j.challenge.Challenge;
@@ -30,7 +29,7 @@ public class Authorization {
 
     private String domain;
     private String status;
-    private Date expires;
+    private String expires;
     private List<Challenge> challenges;
     private List<List<Challenge>> combinations;
 
@@ -65,14 +64,14 @@ public class Authorization {
     /**
      * Gets the expiry date of the authorization, if set by the server.
      */
-    public Date getExpires() {
+    public String getExpires() {
         return expires;
     }
 
     /**
      * Sets the expiry date of the authorization.
      */
-    public void setExpires(Date expires) {
+    public void setExpires(String expires) {
         this.expires = expires;
     }
 
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java
index a5f4006a..2c518bcf 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java
@@ -14,7 +14,6 @@
 package org.shredzone.acme4j.challenge;
 
 import java.net.URI;
-import java.util.Date;
 import java.util.Map;
 
 import org.shredzone.acme4j.Account;
@@ -52,7 +51,7 @@ public interface Challenge {
     /**
      * Returns the validation date, if returned by the server.
      */
-    Date getValidated();
+    String getValidated();
 
     /**
      * Authorizes a {@link Challenge} by signing it with an {@link Account}. This is
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/GenericChallenge.java b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/GenericChallenge.java
index 31c66acd..968e462a 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/GenericChallenge.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/GenericChallenge.java
@@ -19,7 +19,6 @@ import java.net.URISyntaxException;
 import java.security.Key;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
-import java.util.Date;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -73,7 +72,7 @@ public class GenericChallenge implements Challenge {
     }
 
     @Override
-    public Date getValidated() {
+    public String getValidated() {
         return get(KEY_VALIDATED);
     }