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Difference between 2 dates in days and in double

I am working on an app and i need to get the difference between the actual date and a date inserted by the user, in days and in double.
Any idea on how to make this? I've tried some things but without success.

First you must decide if you want to consider the time of the day and the timezone to calculate the difference, because this can lead to different results.
Example: current date (AKA "today") is April 17th or 18th, depending on where in the world you are. Actually, depending on the time of the day, there might be 3 different "todays" in the world, at the same time. What timezone are you using to calculate the difference?
the user will enter a date: only day, month and year? Will it enter the hours? Are you using the user's device's timezone or some specific zone?
the same questions apply to the current date
Depending on the choices you make, you might get a different result.
Anyway, I'd use this lib: http://www.threeten.org/threetenbp/
or java.time classes, if available in your API level. In both API's you can use the following.
To use a date (day-month-year only) and the device's default timezone, I'd choose the LocalDate class:
// current date in device's default timezone
LocalDate now = LocalDate.now();
// some date from input values (May 10th 2018)
LocalDate dt = LocalDate.of(2018, 5, 10);
// difference in days
long diff = ChronoUnit.DAYS.between(now, dt); // 23
If you want to consider the time of the day (hours, minutes, etc), use a LocalDateTime. But in this case, ChronoUnit.DAYS considers a day has passed when the time is >= the other (ex: the difference between April 17th at 10 AM and April 18th 9:59 AM is zero days, because the time of the day didn't reach 10 AM, so it didn't complete 1 day - with LocalDate this doesn't happen because this class doesn't have time-of-the-day fields and considers only the day, month and year).
If you want to consider everything (date, time, and timezone), including Daylight Saving Time transitions, use a ZonedDateTime instead (the code is very similar, the only difference is that you can choose a timezone to work with):
// current date/time in device's default timezone
ZonedDateTime now = ZonedDateTime.now(ZoneId.systemDefault());
// some date from input values (May 10th 2018, 10 AM in New York timezone)
ZonedDateTime dt = ZonedDateTime.of(2018, 5, 10, 10, 0, 0, 0, ZoneId.of("America/New_York"));
// difference in days
long diff = ChronoUnit.DAYS.between(now, dt); // 23
You can choose between the device's default timezone (ZoneId.systemDefault()) or a specific one (ZoneId.of("America/New_York")). You can check all the available timezones with ZoneId.getAvailableZoneIds().
Maybe it doesn't make sense to use current date in one timezone and user's date in another (I'd use the same for both), but that's up to you to decide.

Calendar c = Calendar.getInstance();
Calendar c2 // = what you will get from the user
long diff = c.getTimeInMillis()-c2.
double days = (double) diff/(1000*60*60*24);
that is what i have in mind.
I hope this helps

use this way
public static double getTimeDiffBetweenDate(Date startDateTime, Date finishDateTime) {
long diffInMilliseconds = finishDateTime.getTime() - startDateTime.getTime();
return TimeUnit.MILLISECONDS.toMinutes(diffInMilliseconds) / 60.0;
}

Sort a column with string format dd/MM/yyyy in SQLite (Android)

I have an SQLite database in an Android project with a Date column that stores the date-time as String in dd-mm-yyyy HH-mm-ss format. I need to sort it based on the descending order of the date.
Or, convert it to the standard yyyy-MM-dd HH-mm-ss format and then sort it.
The general ORDER BY DATETIME(coulmn_name) doesnt work.
NOTE:
This is not a duplicate question, other answers advice to change the database schema (Which is not possible, because I have data stored already)

I would like to suggest an alternative approach to the one you are taking. I personally ran into the same issue and solved it by not using a string date at all.
Instead i converted the date to epoch milliseconds ie unix timestamp and saved that. Then a sort is a simple order by the timestamp.
You can use the following approach:
SimpleDateFormat sdf = new SimpleDateFormat ("dd-MM-yyyy HH:mm:ss");
Date currentDate = sdf.parse(yourdatestring);
//Get the calendar in the time zone you need, generally it works off the bat with the default time zone
Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("yourtz"));
cal.setTime(currentDate);
//Get the milliseconds since epoch time
long millis = cal.getTimeInMillis();
You can save this timestamp and easily sort it. It'll be more accurate and easy to use than a string and potentially gives you the ability to handle different time zones.
You can retrieve the date by setting this timestamp directly in the calendar and getting a date from it
cal.setTimeInMillis(timestamp).getTime();
Hope this helps

Get Week from given Calendar Range?

I am implementing an application where I have two calendar object with minimum and maximum date range.
Is there a way to get total week count
How can get week information based on index say: need 5th week of calendar range
any one have suggestion here ?

As mentioned here Get the number of weeks between two Dates. maybe you can use JodaTime http://www.joda.org/joda-time/
android version https://github.com/dlew/joda-time-android
DateTime dateTime1 = new DateTime(date1);
DateTime dateTime2 = new DateTime(date2);
int weeks = Weeks.weeksBetween(dateTime1, dateTime2).getWeeks();

Android inconsistency when dates are stored as longs and the time zone is changed

I have an Android app that is used by people in the UK and Ireland only, and there are no plans for this app to be used overseas.
In the app I store dates for various things, although I never need the full timestamp including the time of day, I only need the date.
So that I can compare dates easily, I've been creating calendar objects and clearing the values of the time, and using the milliseconds of that to store in the database.
public Calendar clearCalTime(long l) {
Calendar cal = Calendar.getInstance();
cal.setFirstDayOfWeek(Calendar.MONDAY);
cal.setTimeInMillis(l);
cal.clear(Calendar.HOUR);
cal.clear(Calendar.HOUR_OF_DAY);
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);
return cal;
}
The issue has been that if users change the timezone, for some reason the dates start messing up, e.g by saving things to the wrong day. The timezones could be anything, I have no control over what the users set.
I've tried setting the timezone to UTC but this doesn't work either. Is there any way to just disregard the timezones?

Nope. You are going to have to set TimeZone going in and out of your storage.
You'll need to do this:
Calendar c = new GregorianCalendar(TimeZone.getTimeZone("GMT"));
in your code to make it consistent.
See this SO: Java.util.Calendar - milliseconds since Jan 1, 1970
Alternatively use Joda Time when dealing with complicated Date/Time math.

Calendar dayOfMonth off by 1

When trying to get a string for the current date using
DateFormat.getDateInstance().format(calendar.getTime())
it keeps returning the wrong day. For example, it is saying today, July 25th., is July 26th. Also when I use it to sat a date picker, I get the day value by using
dayOfMonth = calendar.get(Calendar.DAY_OF_MONTH);
When the date picker is set, it also shows the day ahead by 1.
To get the calendar I'm using
Calendar calendar = Calendar.getInstance();
Is there something I'm missing?
Thanks

I would imagine this is because you havent set the timezone to your timezone, and rather than the day being off randomly, the time zone you are in is diferent than GMT (Greenwich Median? Time). Try looking at this example How to handle calendar TimeZones using Java?