Calendar : don't allow past timestamp

Question

I'm scheduling a repeating alarm, ringing every Tuesday morning, at 08:30.

Because of Doze, I can't use AlarmManager.setRepeating because the time of the alarm won't be precise (it needs to be, I understand the effects on battery).

So, I use this code to schedule the first alarm :

Calendar calendar = Calendar.getInstance();
calendar.setTimeInMillis(System.currentTimeMillis());
calendar.setFirstDayOfWeek(Calendar.WEDNESDAY); // Trick not to get a "tuesday" timestamp in the past if today is wednesday+ !
calendar.set(Calendar.DAY_OF_WEEK, Calendar.TUESDAY);
calendar.set(Calendar.HOUR_OF_DAY, 8);
calendar.set(Calendar.MINUTE, 30);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MILLISECOND, 0);
long scheduledTime = calendar.getTimeInMillis();

AlarmManager alarmManager = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
    alarmManager.setExactAndAllowWhileIdle(AlarmManager.RTC_WAKEUP, scheduledTime, pendingIntent);
} else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
    alarmManager.setExact(AlarmManager.RTC_WAKEUP, scheduledTime, pendingIntent);
} else {
    alarmManager.set(AlarmManager.RTC_WAKEUP, scheduledTime, pendingIntent);
}

And it works flawlessly.

Problem is, I have to re-schedule the alarm when the Intent in consumed by my BroadcastReceiver, on Tuesday, 8:30AM.

But between Tuesday 08:30 and Tuesday 23:59, the method will give me a timestamp in the "past", which is incorrect.

Is there any method than "if the provided timestamp is in the past, add 1 week to it" to fix this issue?

Answer 1

I would suggest going for the new java.time API for this calculation. The ThreeTenABP project is an adaptation of that library for Android specifically.

Here is an example of getting 08:30 AM of every Tuesday using java.time API:

LocalDateTime now = LocalDateTime.now(ZoneId.systemDefault());
now.with(TemporalAdjusters.next(DayOfWeek.TUESDAY))
   .withHour(8)
   .withMinute(30)
   .withSecond(0));  

Answer 2

The old classes (Date, Calendar and SimpleDateFormat) have lots of problems and design issues, and they're being replaced by the new APIs.

In Android you can use the ThreeTen Backport, a great backport for Java 8's new date/time classes, together with the ThreeTenABP (more on how to use it here).

Although you can also use Joda-Time, it's in maintainance mode and being replaced by the new APIs, so I don't recommend start a new project with it. Even in joda's website it says: "Note that Joda-Time is considered to be a largely “finished” project. No major enhancements are planned. If using Java SE 8, please migrate to java.time (JSR-310).".

All the classes below are under the package org.threeten.bp.

---

First of all, I've checked the documentation of AlarmManager and it seems to work with timestamps (the long parameter in set methods), which is the number of milliseconds from 1970-01-01T00:00Z.

So, you need a ZonedDateTime (a date and time in a specific timezone), because you need to know the date, time and the timezone to get a specific timestamp millis:

import org.threeten.bp.DayOfWeek;
import org.threeten.bp.LocalTime;
import org.threeten.bp.ZonedDateTime;
import org.threeten.bp.temporal.TemporalAdjusters;

// current date/time at system's default timezone
ZonedDateTime now = ZonedDateTime.now();

// get next Tuesday at 08:30 AM
ZonedDateTime nextTuesday = now
    // get the next Tuesday
    .with(TemporalAdjusters.next(DayOfWeek.TUESDAY))
    // at 08:30 AM
    .with(LocalTime.of(8, 30));
System.out.println(nextTuesday);

// get the millis timestamp
long scheduledTime = nextTuesday.toInstant().toEpochMilli();

The TemporalAdjusters.next() method does the trick of finding the next Tuesday. If the current date is already a Tuesday and you don't want to get the Tuesday of next week, you can replace it by TemporalAdjusters.nextOrSame().

The LocalTime.of(8, 30) sets the hour to 08:30 AM (it already sets the seconds and milliseconds to zero).

---

The code above gets the current date/time using the system's default timezone (probably the one configured in the device). If you want a specific timezone, you can use the org.threeten.bp.ZoneId or org.threeten.bp.ZoneOffset classes:

// current date/time at London
ZonedDateTime now = ZonedDateTime.now(ZoneId.of("Europe/London"));

// current date/time in UTC
ZonedDateTime now = ZonedDateTime.now(ZoneOffset.UTC);

Note that the API uses IANA timezones names (always in the format Continent/City, like America/Sao_Paulo or Europe/London). Avoid using the 3-letter abbreviations (like CST or PST) because they are ambiguous and not standard.

---

If you already have a java.util.Calendar, you can convert from and to the new API using the org.threeten.bp.DateTimeUtils class:

// convert calendar to ZonedDateTime, using system's default timezone
ZonedDateTime z = DateTimeUtils.toInstant(calendar).atZone(ZoneId.systemDefault());

// convert back to calendar
Calendar c = DateTimeUtils.toGregorianCalendar(z);

Answer 3

You can calculate next Tuesday's 8:30 am with this code. It uses JodaTime Library.

At the time of this answer this is the dependancy to add to build.gradle: compile 'joda-time:joda-time:2.9.9'

public DateTime getNextTuesday830(DateTime now){
        DateTime rtn = now;

        int thisDay = now.getDayOfWeek();
        int deltaNexTuesday = -1;

        if(thisDay >= DateTimeConstants.TUESDAY){
            deltaNexTuesday =  DateTimeConstants.TUESDAY + 7 - thisDay;
        }else{
            deltaNexTuesday = DateTimeConstants.TUESDAY - thisDay;
        }

        rtn = rtn.plusDays(deltaNexTuesday).withHourOfDay(8).withMinuteOfHour(30).withSecondOfMinute(0);

        return rtn;
    }

To calculate the now parameter:

DateTime now = new DateTime(System.currentTimeMillis());

To get back a Date() object from DateTime:

Date d = now.toDate();