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calculating timestamp operations

hi guys

I have this code

I am trying to get the currentTime at which my program runs and want to get 7 hours prior to that


DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS"); 
Calendar cal = Calendar.getInstance();  
String currentRunningTime = dateFormat.format(cal.getTime()); //current time at which my program runs
String sevenHoursPreviousTime = "";

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so if currentRunningTime =  2013-11-19 13:21:55.027
i want to subtract 7 hours from currentRunningTime.

Any idea how i can do that?
Any help would be appreciated.

thanks
0
royjayd
Asked:
royjayd
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3 Solutions
 
dpearsonCommented:
You want something like this:

Calendar cal =       cal.add(Calendar.HOUR_OF_DAY, -7) ;      // 7 hours ago

Doug
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CEHJCommented:
Try

	Calendar c = Calendar.getInstance();
	c.add(Calendar.HOUR, -7);
	String sevenEarlier = new java.sql.Timestamp(c.getTime().getTime()).toString();
	System.out.println(sevenEarlier);

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0
 
jb1devCommented:
As Doug suggests, you can use the calendar class.

You can also do it without making use of the Calendar class by calculating the milliseconds yourself.

E.g.
import java.util.*;
import java.text.*;

public class Time {
    
    public static void main(String[] args) {
        DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
        String currentRunningTime = dateFormat.format(System.currentTimeMillis());
        String sevenHoursPreviousTime = dateFormat.format(
                    System.currentTimeMillis() - 
                    (1000 * 60 * 60 * 7) );

        System.out.println(currentRunningTime);
        System.out.println(sevenHoursPreviousTime);

    }

}

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jb1devCommented:
Also, I would highly recommend against using the java.sql.Timestamp class if you are not working directly with SQL in some kind of persistence layer.
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jb1devCommented:
Sorry, it is more accurate to call the System.currentTimeMillis() function only once (otherwise you are off by a few milliseconds)
This is better.

import java.util.*;
import java.text.*;

public class Time {
    
    public static void main(String[] args) {
        DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
        long currentTimeMillis = System.currentTimeMillis();
        String currentRunningTime = dateFormat.format(currentTimeMillis);
        String sevenHoursPreviousTime = dateFormat.format(
                    currentTimeMillis - (1000 * 60 * 60 * 7) );

        System.out.println(currentRunningTime);
        System.out.println(sevenHoursPreviousTime);

    }

}

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0
 
CEHJCommented:
Also, I would highly recommend against using the java.sql.Timestamp class if you are not working directly with SQL in some kind of persistence layer.
Why is that?
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jb1devCommented:
Why would you want to use java.sql.* classes outside of the context of JDBC?

This is a great way to introduce bugs when someone has a java.util.Date and they want to compare against a java.sql.Timestamp (which is a subclass and could be passed around as the base class, hiding it's true type from the user.)

From the javadoc:
http://docs.oracle.com/javase/7/docs/api//java/sql/Timestamp.html

Note: This type is a composite of a java.util.Date and a separate nanoseconds value. Only integral seconds are stored in the java.util.Date component. The fractional seconds - the nanos - are separate. The Timestamp.equals(Object) method never returns true when passed an object that isn't an instance of java.sql.Timestamp, because the nanos component of a date is unknown. As a result, the Timestamp.equals(Object) method is not symmetric with respect to the java.util.Date.equals(Object) method. Also, the hashCode method uses the underlying java.util.Date implementation and therefore does not include nanos in its computation.

Due to the differences between the Timestamp class and the java.util.Date class mentioned above, it is recommended that code not view Timestamp values generically as an instance of java.util.Date. The inheritance relationship between Timestamp and java.util.Date really denotes implementation inheritance, and not type inheritance.
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CEHJCommented:
Why would you want to use java.sql.* classes outside of the context of JDBC?
Because it's a smple tool to do a simple job - namely to get a timestamp as a String in the format that royjayd needs. Your observations on type complexity do not seem to be relevant to this case
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