Parse legacy timestamp strings without timezone information in them and adapt them according to known timezone, considering daylight saving time.

TimestampParser.java

import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

// parse timestamp strings not containing timezone information (but timezone is known)
// without daylight saving time this would be easy (add fixed string like "+01:00")
// how to use Java 8 time API?
// with Java 11+ simply run as: java TimestampParser.java
public class TimestampParser
{
  /** test cases legacy timestamp to be converted, do not contain timezone
   in Europe/Berlin in the year 2021 daylight saving time (DST)
   is between Mar 28th 3am and Oct 31st 3am */
  public static final String[] INPUT_BERLIN =
  {//YYYY-MM-DD,hh:mm:ss,millis
    "2021-03-28,01:00:00,123", // before DST, switches at 2am to 3am
    "2021-03-28,02:00:49,123", // not valid, does not exist
    "2021-03-28,03:00:49,123", // first hour of DST, one hour after first timestamp
                               // at 3am time switches back to 2am
    "2021-10-31,02:00:49,123", // ambiguous: first or second time 2:xx am?
    "2021-10-31,03:00:49,123", // after DST ended
  };
  /** expected output when reformatting parsed timestamps with ISO-8601 */
  public static final String[] OUTPUT_BERLIN =
  {//ISO-8601
    "2021-03-28T01:00:00.123+01:00",
    null,
    "2021-03-28T03:00:49.123+02:00",
    "2021-10-31T02:00:49.123+01:00", // arbitrary pick, might have been +02:00
    "2021-10-31T03:00:49.123+01:00",
  };
  public static final String[] INPUT_MEMPHIS =
  {//YYYY-MM-DD,hh:mm:ss,millis
    "2021-03-14,01:00:00,123", // before DST, switches at 2am to 3am
    "2021-03-14,02:00:49,123", // not valid, does not exist
    "2021-03-14,03:00:49,123", // first hour of DST, one hour after first timestamp
                               // at 3am time switches back to 2am
    "2021-11-07,01:00:49,123", // ambiguous: first or second time?
    "2021-11-07,02:00:49,123", // after DST ended
  };
  /** expected output when reformatting parsed timestamps with ISO-8601 */
  public static final String[] OUTPUT_MEMPHIS =
  {//ISO-8601
    "2021-03-14T06:00:00.123-06:00",
    null,
    "2021-03-14T08:00:49.123-05:00",
    "2021-11-07T08:00:49.123-05:00", // arbitrary pick, might have been +02:00
    "2021-11-07T08:00:49.123-06:00",
  };

  public static void parse(final String[] INPUT, final String[] OUTPUT, ZoneId zone)
  {
    System.out.println("Parse for " + zone.getId() + " on a system with default " + ZoneId.systemDefault().getId());
    DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd,HH:mm:ss,SSS");
    int i = 0;
    for (String t : INPUT)
    {
      LocalDateTime ldt = LocalDateTime.parse(t, dtf);
      ZonedDateTime withZone = ZonedDateTime.of(ldt, zone);
      System.out.println("input       =" + t);
      System.out.println("parsed local=" + ldt.toString());
      System.out.println("parsed zone =" + withZone);
      System.out.println("expected    =" + OUTPUT[i++]);
      System.out.println("parsed utc  =" + withZone.toInstant());
      System.out.println();
    }
  }

  public static void main(String[] args)
  {
    parse(INPUT_BERLIN, OUTPUT_BERLIN, ZoneId.of("Europe/Berlin"));
    parse(INPUT_MEMPHIS, OUTPUT_MEMPHIS, ZoneId.of("America/Chicago"));
  }
}