How to get the first day of the current week and month?

Question

I have the date of several events expressed in milliseconds*, and I want to know which events are inside the current week and the current month, but I can't figure out how to obtain the first day (day/month/year) of the running week and convert it to milliseconds, the same for the first day of the month.

*Since January 1, 1970, 00:00:00 GMT

Answer 1

You should be able to convert your number to a Java Calendar, e.g.:

 Calendar.getInstance().setTimeInMillis(myDate);

From there, the comparison shouldn't be too hard.

Answer 2

The first day of week can be determined with help of java.util.Calendar as follows:

Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.setTimeInMillis(timestamp);
while (calendar.get(Calendar.DAY_OF_WEEK) > calendar.getFirstDayOfWeek()) {
    calendar.add(Calendar.DATE, -1); // Substract 1 day until first day of week.
}
long firstDayOfWeekTimestamp = calendar.getTimeInMillis();

The first day of month can be determined as follows:

Calendar calendar = Calendar.getInstance();
calendar.clear();
calendar.setTimeInMillis(timestamp);
while (calendar.get(Calendar.DATE) > 1) {
    calendar.add(Calendar.DATE, -1); // Substract 1 day until first day of month.
}
long firstDayOfMonthTimestamp = calendar.getTimeInMillis();

Pretty verbose, yes.

---

Java 7 will come with a much improved Date and Time API (JSR-310). If you can't switch yet, then you can as far use JodaTime which makes it all less complicated:

DateTime dateTime = new DateTime(timestamp);
long firstDayOfWeekTimestamp = dateTime.withDayOfWeek(1).getMillis();

and

DateTime dateTime = new DateTime(timestamp);
long firstDayOfMonthTimestamp = dateTime.withDayOfMonth(1).getMillis();

Answer 3

This week in milliseconds:

// get today and clear time of day
Calendar cal = Calendar.getInstance();
cal.clear(Calendar.HOUR_OF_DAY);
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

// get start of this week in milliseconds
cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
System.out.println("Start of this week:       " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

// start of the next week
cal.add(Calendar.WEEK_OF_YEAR, 1);
System.out.println("Start of the next week:   " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

This month in milliseconds:

// get today and clear time of day
Calendar cal = Calendar.getInstance();
cal.clear(Calendar.HOUR_OF_DAY);
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

// get start of the month
cal.set(Calendar.DAY_OF_MONTH, 1);
System.out.println("Start of the month:       " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

// get start of the next month
cal.add(Calendar.MONTH, 1);
System.out.println("Start of the next month:  " + cal.getTime());
System.out.println("... in milliseconds:      " + cal.getTimeInMillis());

Answer 4

In this case:

// get today and clear time of day
Calendar cal = Calendar.getInstance();
cal.clear(Calendar.HOUR_OF_DAY);  <---- is the current hour not 0 hour
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

So the Calendar.HOUR_OF_DAY returns 8, 9, 12, 15, 18 as the current running hour. I think will be better change such line by:

c.set(Calendar.HOUR_OF_DAY,0);

this way the day always begin at 0 hour