use strtoul for conversion

src/zm_crypt.cpp

@@ -43,7 +43,11 @@ std::pair <std::string, unsigned int> verifyToken(std::string jwt_token_str, std
     }
 
      if (decoded.has_payload_claim("iat")) {
-      token_issued_at  = (unsigned int) (decoded.get_payload_claim("iat").as_int());
+
+      
+      std::string iat_str = decoded.get_payload_claim("iat").as_string();
+      Info ("Got IAT token=%s", iat_str);
+      token_issued_at  = strtoul(iat_str, NULL,0 );
     } 
     else {
       Error ("IAT not found in claim. This should not happen");