how to tell assertJsonEquals to ignore one field in comparing

You could use the library https://github.com/skyscreamer/JSONassert that has an assert method that allows customization.

Here's an example of a test that is passing (and such ignoring the value of the time field)

  @Test
  public void test() throws JSONException {
    JSONAssert.assertEquals("{x: 1, time:123}",
                            "{x: 1, time:234}",
                            new CustomComparator(
                              JSONCompareMode.STRICT,
                              Customization.customization("time", // json path you want to customize
                                                          new ValueMatcher() {
                                @Override public boolean equal(Object o1, Object o2) {
                                  return true; // in your case just ignore the values and return true
                                }
                              }))
    );
  }

Here's the link to the javadoc of the assertEquals method that I am using in the example: http://jsonassert.skyscreamer.org/apidocs/org/skyscreamer/jsonassert/JSONAssert.html#assertEquals-java.lang.String-java.lang.String-org.skyscreamer.jsonassert.comparator.JSONComparator-