如何在Java中将当前时间添加到上一个日期？

Question

I was trying to add current time into previous date. But it was adding in current date with time not with previous date.

see my bellow code:

Date startUserDate = ;//this is my previous date object;
startUserDate.setTime(new Date().getTime());// here i'm trying to add current time in previous date.
System.out.println("current time with previous Date :"+startUserDate);

In previous date there is no time and i want to add current time in previous date.I can do this, please help me out.

Answer 1

Use calendar object

Get instance of calendar object and set your past time to it Date startUserDate = ;

    Calendar calendar = Calendar.getInstance();
    calendar.settime(startUserDate);

Create new calendar instance

     Calendar cal = Calendar.getInstance();
     cal.settime(new Date());

format the date to get string representation of time of current date

        SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
        String currentdate =  sdf.format(cal.getTime());

split that string to get hour minute and second object

                String hh = expiry.split(":")[0];
                String mm = expiry.split(":")[1];
                String ss = expiry.split(":")[2];

add it to the previous calendar object

    calendar .add(Calendar.HOUR_OF_DAY, hh);
    calendar .add(Calendar.MINUTE, mm);
    calendar .add(Calendar.SECOND, ss);

this date will have current time added to your date

   Date newDate = calendar.getTime;

Answer 2

You can do it using DateFormat and String , here's the solution that you need:

Code:

DateFormat df = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");
String timeString = df.format(new Date()).substring(10); // 10 is the beginIndex of time here

DateFormat df2 = new SimpleDateFormat("MM/dd/yyyy");
String startUserDateString = df2.format(startUserDate);

startUserDateString = startUserDateString+" "+timeString;
// you will get this format "MM/dd/yyyy HH:mm:ss" 

//then parse the new date here
startUserDate = df.parse(startUserDateString);

Explanation:

Just convert the current date to a string and then extract the time from it using .substring() method, then convert your userDate to a string concatenate the taken time String to it and finally parse this date to get what you need.

Example:

You can see it working in this ideone DEMO .

Which takes 02/20/2002 in input and returns 02/20/2002 04:36:14 as result.

Answer 3

Use Calendar :

• first set the date/time of the first calendar object to the old date

• object use as second Calendar object to set the current time on the

• first calendar object then convert it back to date

as follow:

//E.g. for startUserDate 
Date startUserDate = new Date(System.currentTimeMillis() - (24L * 60L * 60L * 1000L) - (60L * 60L * 1000L));//minus 1 day and 1 hour
Calendar calDateThen = Calendar.getInstance();
Calendar calTimeNow = Calendar.getInstance();
calDateThen.setTime(startUserDate);
calDateThen.set(Calendar.HOUR_OF_DAY, calTimeNow.get(Calendar.HOUR_OF_DAY));
calDateThen.set(Calendar.MINUTE, calTimeNow.get(Calendar.MINUTE));
calDateThen.set(Calendar.SECOND, calTimeNow.get(Calendar.SECOND));
startUserDate = calDateThen.getTime();
System.out.println(startUserDate);

The second Calendar object calTimeNow can be replaced with Calendar.getInstance() where it is used.

Answer 4

The getTime method returns the number of milliseconds since 1970/01/01 so to get the time portion of the date you can either use a Calendar object or simply use modula arithmetic (using the above milliseconds value and the MAX millseconds in a day) to extract the time portion of the Date.

Then when you have the time you need to add it to the second date,

but seriously, use http://docs.oracle.com/javase/7/docs/api/java/util/Calendar.html

and use things like get (HOUR) and get (MINUTE) etc. which then you can use with set (HOUR, val)

Answer 5

You need to use Calendar class to perform addition to Date object. Date's setTime() will set that time in Date object but not add ie it will overwrite previous date. new Date().getTime() will not return only time portion but time since Epoch. Also, how did you manipulated , startUserDate to not have any time (I mean , was it via Calendar or Formatter) ?

See Answer , Time Portion of Date to calculate only time portion,

long MILLIS_PER_DAY = 24 * 60 * 60 * 1000;
Date now = Calendar.getInstance().getTime();
long timePortion = now.getTime() % MILLIS_PER_DAY;

then you can use something like, cal.add(Calendar.MILLISECOND, (int)timePortion); where cal is Calendar object corresponding to your startUserDate in your code.