如何为我已经拥有的时间戳添加分钟？

Question

I searched a lot, but I found a way to add or subtract time from the calendar instance which gives the current time.. how do I subtract time from the last modified time of a given file?

UPDATE : I've been using Java1.4. That is the reason I'm unable to find any methods to do this. I extracted the modified date as a string. I wanted to convert this string obey I a calendar object so that it's easier for me to apply add () of the calendar object to the time. I've been facing issues with the same. is this approach correct? Could you please assist

Answer 1

If you dont have to use calendar, than just use the new Java DateTime API available since Java8 https://docs.oracle.com/javase/8/docs/api/java/time/package-summary.html

There you have very nice convenience methods like plus/minus etc.

For example you can simply write

  LocalTime now = LocalTime.now();
  now.minusHours(2);

Answer 2

Timestamp (as long ) you can set in Calendar instance. And after that you can add() time.

public void time() {
    long timeStamp = 31415926535L;

    Calendar calendar = Calendar.getInstance();

    calendar.setTimeInMillis(timeStamp);

    // Substract 1 hour
    calendar.add(Calendar.HOUR, -1);

    // Add 20 minutes
    calendar.add(Calendar.MINUTE, 20);
}

Answer 3

NIO and java.time

    Path filePath = Paths.get("myFile.txt");
    Duration timeToSubtract = Duration.ofMinutes(7);

    FileTime lastModified = Files.getLastModifiedTime(filePath);
    Instant lastModifiedInstant = lastModified.toInstant();
    Instant timeBeforeLastModified = lastModifiedInstant.minus(timeToSubtract);
    System.out.println("Time after subtraction is " + timeBeforeLastModified);

Running just now on my computer I got this output:

Time after subtraction is 2017-02-18T03:06:04Z

The Z at the end indicates UTC . Instant::toString (implicitly called when appending the Instant to a string) always generates a string in UTC.

I am using the modern Java NIO API and java.time , the modern Java date and time API. NIO gives us a FileTime in this case denoting the time the file was last modified. In order to do our time math I first convert it to an Instant , which is a central class of java.time. The minus method of an Instant subtracts a Duration , an amount of time, and returns a new Instant object.

Don't use Calendar . That class was poorly designed and is long outdated.

Link: Oracle tutorial: Date Time explaining how to use java.time.

Answer 4

Since Java 8, java.util.Date , java.util.Calendar , and java.text.SimpleDateFormat are now legacy. So I edit my codes, remove the legacy classes.

Thanks @Ole VV and @Basil Bourque for pointing my problems.

---

I'm confused about your issue. I guess you want to modify a file's last modified time.

So I write down the codes.

import java.io.File;
import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneId;

public class Test {

  private static final long ONE_SECOND = 1000L;
  private static final long ONE_MINUTE = 60L * ONE_SECOND;

  public static void main(String[] args) {
    File file = new File("test.txt");
    if (!file.exists()) {
      System.err.println("File doesn't exist.");
      return;
    }

    //get file's last modified, in millisecond
    long timestamp = file.lastModified();
    System.out.println("File's last modified in millisecond: " + timestamp);

    //print time for human, convert to LocalDateTime
    LocalDateTime localDateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(timestamp), ZoneId.systemDefault());
    System.out.println("File's last modified in LocalDateTime: " + localDateTime);

    //add a minute
    timestamp = timestamp + ONE_MINUTE;

    //modify file's last modified
    boolean isModified = file.setLastModified(timestamp);
    if (isModified) {
      System.out.println("Update file's last modified successfully.");
      System.out.println("File's last modified in millisecond: " + file.lastModified());
      //print time for human, convert to LocalDateTime
      System.out.println("File's last modified in LocalDateTime: " +
          LocalDateTime.ofInstant(Instant.ofEpochMilli(file.lastModified()), ZoneId.systemDefault()));
    } else {
      System.err.println("Update file's last modified failed.");
    }
  }

}

Besides, If you want modify a timestamp, just use +/- operations.

And you can convert timestamp to LocalDateTime, and use LocalDateTime's api to modify time easily.

public void modifyTime() {
  //modify timestamp: add one second
  long timestamp = System.currentTimeMillis();
  timestamp = timestamp + 1000L;

  //convert timestamp to LocalDateTime
  LocalDateTime localDateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(timestamp), ZoneId.systemDefault());

  //modify LocalDateTime
  //add a minute
  localDateTime = localDateTime.plusMinutes(1);
  //subtract a second
  localDateTime = localDateTime.minusSeconds(1);
}

If I misunderstood your idea, please let me know.

Answer 5

You can use setTime() like this:

yourTimeStamp.setTime(yourTimeStamp.getTime() + TimeUnit.MINUTES.toMillis(minutesToAdd));