A Gson Serializer/Deserializer for java.util.Date objects.

DateSerializer.java

public class DateTypeDeserializer implements JsonDeserializer<Date>, JsonSerializer<Date> {
    private static final String[] DATE_FORMATS = new String[]{
            "yyyy-MM-dd'T'HH:mm:ssZ",
            "yyyy-MM-dd'T'HH:mm:ss",
            "yyyy-MM-dd",
            "EEE MMM dd HH:mm:ss z yyyy",
            "HH:mm:ss",
            "MM/dd/yyyy HH:mm:ss aaa",
            "yyyy-MM-dd'T'HH:mm:ss.SSSSSS",
            "yyyy-MM-dd'T'HH:mm:ss.SSSSSSS",
            "yyyy-MM-dd'T'HH:mm:ss.SSSSSSS'Z'",
            "MMM d',' yyyy H:mm:ss a"
    };

    @Override
    public Date deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        if (json.isJsonPrimitive()) {
            return new Date(json.getAsLong());
        }
        for (String format : DATE_FORMATS) {
            try {
                return new SimpleDateFormat(format, Locale.US).parse(json.getAsString());
            } catch (ParseException ignored) {
            }
        }
        throw new JsonParseException("Unparseable date: \"" + json.getAsString()
                + "\". Supported formats: \n" + Arrays.toString(DATE_FORMATS));
    }

    @Override
    public JsonElement serialize(Date src, Type typeOfSrc, JsonSerializationContext context) {
        if (false) {
            return new JsonPrimitive(new SimpleDateFormat(DATE_FORMATS[0], Locale.US).format(src));
        } else {
            return new JsonPrimitive(src.getTime());
        }
    }
}

https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html#iso8601timezone