How to round to best known INSTANT_SECONDS value with Java DateTimeFormatter?

Question

I'm using a Java DateTimeFormatter in order to parse dates and datetimes. I construct it with a DateTimeFormatterBuilder dtfb, and then try to get epoch seconds from it as follows:

dtfb.toFormatter().withZone(ZoneId.of("America/New_York"))
      .parse(timeStr)
      .getLong(ChronoField.INSTANT_SECONDS)

However, this only works if timeStr is specific enough to identify one particular second. For example, this works: "1997.10.10 23:45:23", but this does not "1997.10.10", as there's no hour minute second information. I'm wondering if there's a way in Java to systematically "round down" and return the first timestamp that fits the constraints. For example, in this failing case I'd want the answer to be the timestamp at "1997.10.10 00:00:00". Basically just keep filling in zeros in smaller and smaller time units until it's specific enough to give me a second. Is this possible?

Answer 1

I think this might be what you want to do. I tried with ChronoField.SECOND_OF_DAY, but that results in a java.time.DateTimeException: Conflict found: HourOfDay 23 differs from HourOfDay 0 while resolving when using your example with a timestamp.

Note that you must start the building with the formatting string. If you use .parseDefaulting prior to .appendPattern, you get a java.time.format.DateTimeParseException.

import java.time.format.DateTimeFormatterBuilder;
import java.time.temporal.ChronoField;
import java.time.ZoneId;

class Testing {
  public static void main(String[] args) {
    DateTimeFormatterBuilder dtfb =
      new DateTimeFormatterBuilder()
        .appendPattern("yyyy.dd.MM[ HH:mm:ss]")
        .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)
        .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0)
        .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0);

    String timeStr = "1997.10.10";
//    String timeStr = "1997.10.10 00:00:01";
//    String timeStr = "1997.10.10 23:45:23";

    Long instant =
      dtfb.toFormatter().withZone(ZoneId.of("America/New_York"))
          .parse(timeStr)
          .getLong(ChronoField.INSTANT_SECONDS);

    System.out.println(instant);
  }
}