Generate a list of hourly or daily time spans in UTC milliseconds in any timezone

TimeIntervalGenerator.java

package cloud.prefab.server.utils;

import cloud.prefab.domain.Internal;
import com.google.common.base.Preconditions;
import java.time.Instant;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.temporal.ChronoUnit;
import java.util.ArrayList;
import java.util.List;

public class TimeIntervalGenerator {

  public static List<TimeSpan> generate(
    long startUtc,
    long endUtc,
    String timezoneName,
    Internal.TimeInterval timeInterval
  ) {
    Preconditions.checkArgument(
      startUtc <= endUtc,
      "Start time %s must be less than end time %s",
      startUtc,
      endUtc
    );
    // timezone convert so we get correct offset
    // snap/truncate to the time unit
    ZoneId timezone = ZoneId.of(timezoneName);
    ChronoUnit chronoUnit = mapInterval(timeInterval);
    ZonedDateTime zonedStartTime = ZonedDateTime.ofInstant(
      Instant.ofEpochMilli(startUtc),
      timezone
    );
    ZonedDateTime zonedEndTime = ZonedDateTime.ofInstant(
      Instant.ofEpochMilli(endUtc),
      timezone
    );
    ZonedDateTime actualEndTime = zonedEndTime
      .truncatedTo(chronoUnit)
      .plus(1, chronoUnit);
    List<TimeSpan> timeSpans = new ArrayList<>();
    ZonedDateTime currentTime = zonedStartTime.truncatedTo(chronoUnit);
    while (true) {
      ZonedDateTime nextTime = currentTime.plus(1, chronoUnit);
      if (nextTime.isAfter(actualEndTime)) {
        break;
      }
      timeSpans.add(new TimeSpan(currentTime, nextTime));
      currentTime = nextTime;
    }
    return timeSpans;
  }

  public record TimeSpan(ZonedDateTime start, ZonedDateTime end) {}

  private static ChronoUnit mapInterval(Internal.TimeInterval timeInterval) {
    switch (timeInterval) {
      case HOURLY:
        return ChronoUnit.HOURS;
      case DAILY:
        return ChronoUnit.DAYS;
      default:
        throw new IllegalArgumentException(
          "Unrecognized unit %s".formatted(timeInterval)
        );
    }
  }
}