Develop Reference

Android AlarmManager scheduling through time zone or daylight shifts

I have a logic that schedules reminders using AlarmManager. I need to implement the following:
Logic 1: when the user changes time zone, eg he travels from UK (UTC+0) to central Europe (UTC+1), alarms should follow the time zone.
Example, a reminder scheduled at 3PM UTC+0 should fire at 4PM UTC+1
Logic 2: when a time shift occurs, eg time shifts to daylight saving time in spring (from UTC+1 to UTC+2), alarms should keep the original time
Example, a reminder scheduled at 3PM UTC+1 should fire at 3PM UTC+2
How can I achieve this? As of now I have no particular logic in place and all the alarms follow Logic 1. I have found no way to identify when a time shift happens.
Scheduling logic is very simple:
LocalDateTime reminderTime = LocalDateTime.of(...)
ZoneOffset currentOffsetForMyZone = ZoneId.systemDefault().getRules().getOffset(Instant.now());
reminderTime.toInstant(currentOffsetForMyZone).toEpochMilli();
alarmManager.setExact(AlarmManager.RTC_WAKEUP, reminderTime, pendingIntent);

For each alarm store the time of day and the time zone in which it was set. This suffices for firing the alarm at the right time no matter if the user is currently in a different time zone. And Java will take summer time (DST) into account.
Your example times and UTC offsets correspond to standard time in those time zones, so let’s start with an example date in standard time even though it was a couple of days ago now:
LocalTime alarmTime = LocalTime.of(15, 0);
ZoneId alarmTimeZone = ZoneId.of("Europe/London");
// Travel to Paris and see the alarm go off at 4, assuming standard time
ZoneId currentTimeZone = ZoneId.of("Europe/Paris");
Instant actualAlarmTime = LocalDate.of(2021, Month.MARCH, 18)
.atTime(alarmTime)
.atZone(alarmTimeZone)
.toInstant();
ZonedDateTime timeOnLocation = actualAlarmTime.atZone(currentTimeZone);
System.out.format("Scheduled at %s or %d millis, goes off at %s local time%n",
actualAlarmTime, actualAlarmTime.toEpochMilli(), timeOnLocation);
The code prints:
Scheduled at 2021-03-18T15:00:00Z or 1616079600000 millis, goes off at
2021-03-18T16:00+01:00[Europe/Paris] local time
Let’s also try a date in the summer time part of the year. I have changed Paris to London and MARCH to APRIL:
// Stay back home in the UK
ZoneId currentTimeZone = ZoneId.of("Europe/London");
Instant actualAlarmTime = LocalDate.of(2021, Month.APRIL, 18)
.atTime(alarmTime)
.atZone(alarmTimeZone)
.toInstant();
Scheduled at 2021-04-18T14:00:00Z or 1618754400000 millis, goes off at
2021-04-18T15:00+01:00[Europe/London] local time
The basic trick is: don’t use the current offset for the time zone where you set the alarm. Let Java automatically apply the offset for the date and time where the alarm is to go off.

If anyone is interested, the fix was to apply the correct offset for the date and time where the alarm is to go of, as pointed out by Ole. My silly mistake was to apply always the current timezone.
LocalDateTime alarmTime = LocalDateTime.of(...)
ZoneId zone = ZoneId.systemDefault();
ZonedDateTime zoneDateTime = ZonedDateTime.of(alarmTime , zone);
long startAtMillis = zoneDateTime.toInstant().toEpochMilli();
//Fire alarm
notificationAlarm.setExactAndAllowWhileIdle(AlarmManager.RTC_WAKEUP, startAtMillis, pendingIntent);

Change Joda DateTime timezone but don't change time

I am using (maybe incorrectly) a Joda DateTime to store a reminder time in a reminders app. When the timezone changes on the device (because of DST or just moving to a different TZ) I want to be able to reset the reminders to use the current timezone but keep the time.
For example: I set a reminder for Friday May 15th, at 15:00. If the timezone changes before that time, I want to be able to recreate the reminder for Friday May 15th at 15:00 local time.
I have already tried to use withZoneRetainFields but I haven't been able to keep the time unchanged:
new DateTime(reminderTimeMillis, DateTimeZone.forID(previousTimeZone)).withZoneRetainFields(DateTimeZone.getDefault())

Well, that method should basically work:
#Test
public void change_timezone_of_reminder() {
DateTime originalReminderDateTime = LocalDateTime.parse("2015-05-15T15:00").toDateTime(
DateTimeZone.forID("Europe/Berlin"));
assertThat(originalReminderDateTime.toString(), equalTo("2015-05-15T15:00:00.000+02:00"));
long reminderMillis = originalReminderDateTime.getMillis();
DateTime updatedTime = new DateTime(reminderMillis, DateTimeZone.forID("Europe/Berlin"))
.withZoneRetainFields(DateTimeZone.forID("America/New_York"));
assertThat(updatedTime.toLocalDateTime(), equalTo(LocalDateTime.parse("2015-05-15T15:00")));
assertThat(updatedTime.toString(), equalTo("2015-05-15T15:00:00.000-04:00"));
}
So, how are you evaluating that the time field is not changing?
Also, if you are storing the reminder time as millis-after-epoch, you don't need to adjust it for DST shifts, if you're calculating the millis using a full timezone ("Europe/Berlin") rather than a fixed offset.

How android calender setInexactRepeating works?

I am developing small android application. And I want to do something in my application after some minutes. These minutes are not static one these are dynamic ones.
So i am using android calender setInexactRepeating for this.
My code looks like this
Calendar cal = Calendar.getInstance();
// Start something after 4 minutes
cal.add(Calendar.MINUTE, 4);
get_alaram_service().setInexactRepeating(AlarmManager.RTC_WAKEUP,
cal.getTimeInMillis(), 1000*300, get_pendingintent());
So this will work setInexactRepeating after 4 min it will run my pending intent and after that it will keep repeating this for this much amount of time. (1000*300).
So my problem is that in setInexactRepeating 2nd parameter is for at what time I want to start my timer and 3rd parameter for repeating this thing. Now 2nd parameter tales value in milisec. I tried to pass my own value of minutes in milisec like(1000*300) then its not working properly. I don't how its working properly. When I checked cal.getTimeInMillis() it is very big integer number. what is that actually.
Am i doing something wrong need your help thank you...

Although this isn't explicitly stated in the documentation the second parameter (triggerAtMillis) is time in milliseconds since the Epoch. This is what Calendar.getInstance() returns. Calling this method will return the current time. This is a big number, since it is actually the number of milliseconds after 1/1/1970. You then need to add something to it (e.g. 4 minutes) to define when the AlarmManager will first fire.

I need to compare my date and time with mobile date and time?

For a task remainder application i need to compare the mobile current date and time with my task and time.
How this is possible??

convert your task time to long value suppose x1, and get system time via System.currentTimeMillis() sppose x2.
compare them like one compare long values..

Use Calendar class.
Get an instance of Calendar, set hours, minutes, date etc, and then use getTimeInMillis() which you need to compare with System.currentTimeMillis().
Also, you might want to take a look at AlarmManager to actually schedule some action to some moment of time.

Repeat scheduled tasks on selected days in Android sdk alarm manager

I have tried to do this without bothering the experts and have read numerous threads here and on other sites. It is clearly my brain not understanding what needs to be done in order for this to work.
My goal is that the app allows the user to enter a time and one or more days in a week. All of the GUI side and storing of the dates and times I have done, however to get the alarm manager to repeat, lets say every Monday at 14:00 and then can send at 14:02 . I have used the java Calendar object to hold the times and days of the week or even used date and day of the week of the month. These are then , as needed, converted to milliseconds for it to be read in by the alarm manager.
I then have used either the alarm manager set or set repeat methods to repeat the event. All I am able to do is get it to occur once and then if I change the emulator date and time to another Monday nothing happens.
The GUI holds the hours and minutes in required variables and then these are used against the calendar objects.
The alarm manager calls a broadcast receiver for the event to occur.
Please can someone simply give an example on how to set specific days such as Monday , Wednesday Friday. I know that separate alarm managers are needed for each day and at the moment I have just focused on Monday as my main test.
Links viewed:
How can i Repeat the Alarm in android for only Monday, Tuesday and Friday
How to repeat the alarm for "n" days at a particular time
how to repeat alarm after 1 day in android
Managed to figure this out now and so follows my answer:
The following code calculates the remaining days between now and the day needed for the scheduled task. the variable whichday is passed via parameter from the method this code belongs to. In the understanding of this whichday represents days of the week 1 through to 7 where 1 is Sunday , 2 is Monday and so .
//This gets the current day of the week as of TODAY / NOW
int checkcurrentday = getcurtime.get(Calendar.DAY_OF_WEEK);
// This calculates the days between now and the day needed which is represented by whichday.
int numberofdays = Calendar.SATURDAY + whichday - checkcurrentday;
//Now add NOT set the difference of the days to your Calendar object
tMondayOn.add(Calendar.DATE, numberofdays);

Well, you need to first use the Java Calendar API (or Joda!) to figure out when the next monday is. Set the alarm to to that time in milliseconds then use setRepeating and pass in a long that represents the interval of one week.