Schedule a work on a specific time with WorkManager

I might be somewhat late but anyway I did this in order to schedule a WorkRequest at a given time (with an optional short delay). You just need to get the time from a TimePicker of whatever:

public static void scheduleWork(int hour, int minute) {
    Calendar calendar = Calendar.getInstance();
    long nowMillis = calendar.getTimeInMillis();

    if(calendar.get(Calendar.HOUR_OF_DAY) > hour ||
            (calendar.get(Calendar.HOUR_OF_DAY) == hour && calendar.get(Calendar.MINUTE)+1 >= minute)) {
        calendar.add(Calendar.DAY_OF_MONTH, 1);
    }

    calendar.set(Calendar.HOUR_OF_DAY,hour);
    calendar.set(Calendar.MINUTE,minute);

    calendar.set(Calendar.SECOND,0);
    calendar.set(Calendar.MILLISECOND,0);
    long diff = calendar.getTimeInMillis() - nowMillis;

    WorkManager mWorkManager = WorkManager.getInstance();
    Constraints constraints = new Constraints.Builder()
            .setRequiredNetworkType(NetworkType.CONNECTED)
            .build();
    mWorkManager.cancelAllWorkByTag(WORK_TAG);
    OneTimeWorkRequest mRequest = new OneTimeWorkRequest.Builder(NotificationWorker.class)
            .setConstraints(constraints)
            .setInitialDelay(diff,TimeUnit.MILLISECONDS)
            .addTag(WORK_TAG)
            .build();
    mWorkManager.enqueue(mRequest);

}

PeriodicWorkRequests now support initial delays from the version 2.1.0-alpha02. You can use the setInitialDelay method on PeriodicWorkRequest.Builder to set an initial delay. Link Here

Example of schedule at every day at 8:00 am. here I'm using joda time library for time operations.

final int SELF_REMINDER_HOUR = 8;

    if (DateTime.now().getHourOfDay() < SELF_REMINDER_HOUR) {
        delay = new Duration(DateTime.now() , DateTime.now().withTimeAtStartOfDay().plusHours(SELF_REMINDER_HOUR)).getStandardMinutes();
    } else {
        delay = new Duration(DateTime.now() , DateTime.now().withTimeAtStartOfDay().plusDays(1).plusHours(SELF_REMINDER_HOUR)).getStandardMinutes();
    }


    PeriodicWorkRequest workRequest = new PeriodicWorkRequest.Builder(
        WorkerReminderPeriodic.class,
        24,
        TimeUnit.HOURS,
        PeriodicWorkRequest.MIN_PERIODIC_FLEX_MILLIS,
        TimeUnit.MILLISECONDS)
        .setInitialDelay(delay, TimeUnit.MINUTES)
        .addTag("send_reminder_periodic")
        .build();


    WorkManager.getInstance()
        .enqueueUniquePeriodicWork("send_reminder_periodic", ExistingPeriodicWorkPolicy.REPLACE, workRequest);

So far it's not possible to achieve exact times using PeriodicWorkRequest.
An ugly work-around would be using a OneTimeWorkRequest and when it fires, set another OneTimeWorkRequest with a new calculated period, and so on.

Unfortunately, you cannot schedule a work at specific time as of now. If you have time critical implementation then you should use AlarmManager to set alarm that can fire while in Doze to by using setAndAllowWhileIdle() or setExactAndAllowWhileIdle().

You can schedule a work, with onetime initial delay or execute it periodically, using the WorkManager as follows:

Create Worker class:

public class MyWorker extends Worker {
    @Override
    public Worker.WorkerResult doWork() {

        // Do the work here

        // Indicate success or failure with your return value:
        return WorkerResult.SUCCESS;

        // (Returning RETRY tells WorkManager to try this task again
        // later; FAILURE says not to try again.)
    }
}

Then schedule OneTimeWorkRequest as follows:

OneTimeWorkRequest mywork=
        new OneTimeWorkRequest.Builder(MyWorker.class)
        .setInitialDelay(<duration>, <TimeUnit>)// Use this when you want to add initial delay or schedule initial work to `OneTimeWorkRequest` e.g. setInitialDelay(2, TimeUnit.HOURS)
        .build();
WorkManager.getInstance().enqueue(mywork);

You can setup additional constraints as follows:

// Create a Constraints that defines when the task should run
Constraints myConstraints = new Constraints.Builder()
    .setRequiresDeviceIdle(true)
    .setRequiresCharging(true)
    // Many other constraints are available, see the
    // Constraints.Builder reference
     .build();

Then create a OneTimeWorkRequest that uses those constraints

OneTimeWorkRequest mywork=
                new OneTimeWorkRequest.Builder(MyWorker.class)
     .setConstraints(myConstraints)
     .build();
WorkManager.getInstance().enqueue(mywork);

PeriodicWorkRequest can be created as follows:

 PeriodicWorkRequest periodicWork = new PeriodicWorkRequest.Builder(MyWorker.class, 12, TimeUnit.HOURS)
                                   .build();
  WorkManager.getInstance().enqueue(periodicWork);

This creates a PeriodicWorkRequest to run periodically once every 12 hours.