Finding the day in seconds since epoch, modulus not working

Question

I have the following code to try and get the current day in seconds since epoch, by removing any seconds after 00:00:00 on any given day:

public void method(Date date) {

...

long dayDate = date.getTime() - (date.getTime() % 86400L);

...

}

For some reason, dayDate is simply being set to date.getTime(), and the mathematical operators are doing nothing here.

How would I go about fixing this?

Answer 1

Like Ignacio already pointed out, date.getTime() returns the number of milliseconds since January 1st, 1970, so your line should have been:

long dayDate = date.getTime() - (date.getTime() % 86400000L);

Iif you are planning on creating a new Date with dayDate, make sure that it has the right timezone, e.g. it should be in the UTC/GMT timezone. Otherwise, strange things like this could happen:

Date epoch = new Date(0);
System.err.println(epoch);

which gives on my machine Thu Jan 01 01:00:00 CET 1970 because my dates are by default created in the CET (+1) timezone. So if you would use your code and you would create a new Date instance by using the long you calculated, you would end up with a date not at 0:00 on that day, but at 1:00.

However, without immediately resorting to Joda Time (which may not be an option in your project), you can use a Calendar to get the number of seconds:

// Create the calendar and set the date.
Calendar calendar = new GregorianCalendar();
calendar.setTime(date);

// Set the hours, minutes, etc. to 0.
calendar.set(Calendar.HOUR, 0);
calendar.set(Calendar.MINUTE, 0);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MILLISECOND, 0);
calendar.set(Calendar.AM_PM, Calendar.AM);

long dayDate = calendar.getTime();

Answer 2

I'll recommend that you use Joda Time library. It has most of the functions related to date, time and calendar that you'll ever need.