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how does visual foxpro handle date calculation? (comming from java) result for date()-day(date())+1 operation?

what is the correct value for the formula date()-day(date())+1?
if date() returns '2016/5/5'
is it, 2016/5/1? or 2016/4/29?
because when converting code from visual foxpro to java?
following code produces different result.
Calendar today2 = Calendar.getInstance();
Calendar endDate = Calendar.getInstance();
endDate.add(Calendar.DATE, -1 * today2.get(Calendar.DATE));
endDate.add(Calendar.DATE, 1);
above code produces 2016/5/1, while:
Calendar today2 = Calendar.getInstance();
today2.add(Calendar.DATE, 1);
Calendar endDate = Calendar.getInstance();
endDate.add(Calendar.DATE, -1 * today2.get(Calendar.DATE));
above code produces 2016/4/29.
not sure which conversion is correct?

Actually it is obvious and 2016/5/1. Math is simple:
date() - day(date()) would be 2016/5/5 - 5 days which is 2016/4/30 (theDate - day(theDate) gives last day of previous month, adding 1 gives first day of month that theDate is in). Adding 1 day to it means 2016/5/1.
I don't know Java, but to me your 2nd Java code is wrong.
In first one, you are subtracting day of month and then adding 1 (same as what VFP is doing).
In second one, you are setting today2 to "tomorrow", then subtracting tomorrow's day of month from today's date. Which would mean date() - (day(date()+1)) and you would get the day before last month's end date.
Update: I think you can simplify your code as:
Calendar today = Calendar.getInstance();
today.add(Calendar.DATE, 1 - today2.get(Calendar.DATE));
IOW the VFP code to find start of a month:
firstDayOfMonth = theDate - day(theDate) + 1
should translate to:
Calendar firstDayOfMonth = Calendar.getInstance();
firstDayOfMonth.add(Calendar.DATE, 1 - firstDayOfMonth.get(Calendar.DATE));

Determine which day of week is each date of the month

I want to create a calendar with Java 8. So far I have this:
YearMonth yearMonthObject = YearMonth.of(year, month);
int daysOfCurrentMonth = yearMonthObject.lengthOfMonth();
int i = 1;
ArrayList<Integer> Dayes = new ArrayList<Integer>();
for(i=1; i<=daysOfCurrentMonth; i++){
Dayes.add(i);
}
Dayes.forEach(value -> System.out.print(value));
which prints the days of the current month (for example May).
How can I determine that 1 is Sunday, 2 is Monday, 3 is Tuesday, ..., 8 is Sunday (next week), etc.?

You have a YearMonth object. For each day of the month, you can call atDay(dayOfMonth) to return a LocalDate at that specific day of month. With that LocalDate, you can then call:
getDayOfMonth() to get back the day of the month as an int;
getDayOfWeek() to get the day of the week as a DayOfWeek. This is an enumeration of all the days of the week.
As such, you should change your Dayes list to hold LocalDates instead of Integers, and then you can have, for example:
YearMonth yearMonthObject = YearMonth.of(year, month);
int daysOfCurrentMonth = yearMonthObject.lengthOfMonth();
ArrayList<LocalDate> dayes = new ArrayList<LocalDate>();
for(int i = 1; i <= daysOfCurrentMonth; i++){
dayes.add(yearMonthObject.atDay(i));
}
dayes.forEach(value -> System.out.println(value.getDayOfMonth() + " " + value.getDayOfWeek()));
This will print each day of that month followed by the corresponding day of the week.
As a side-note, you can get a real display value for the day of week (instead of the name() of the enum like above) by calling getDisplayName(style, locale). The style represents how to write the days (long form, short form...) and the locale is the locale to use for the display name. An example would be:
value.getDayOfWeek().getDisplayName(TextStyle.FULL, Locale.ENGLISH)
which would output the full text of the day of the week in English. Sample output for 04/2016 with the above change:
1 Friday
2 Saturday
3 Sunday
4 Monday
5 Tuesday

This may be a bit of a 'hack' solution, but if you are trying to make a calendar for any year, you may have to use an 'anchor date' (Such as January 1, 1800 as a Wednesday). You then could calculate the number of days that happened between January 1st, 1800 and your current year/month/day. Once you figured out how many days have passed, using Modular 7 you could determine what day it is, and then populate the calendar for the month from there.

Finding out what day the first of any given month is

So I need to make a basic calendar which displays the calendar for a specific year and month (the user should be able to select
any month and any year based on text input).
So far, I have managed to create a calender obj, use Scanner to get the desired month and year from the user but my question is that how do I find out what the first day of the month is? In the example above, it's a Saturday. My logic to building it is that if I know the first day, I can make a String[][] array and start displaying the day on the relevant date by looping through the month from the first day. I've used a scanner to get the required month and year. I then created a calendar object and set the Calendar variables; Calendar.YEAR and Calendar.MONTH, as per required by the user:
Calendar cal= Calendar.getInstance();
cal.set(Calendar.YEAR, year); //my year variable is set 2015
cal.set(Calendar.MONTH, chosenMonth); //my chosenMonth is set to 5 since January starts from 0.
I tried using the following code to test out my calender to see if it will execute the code if Monday was the first day of the month. On june 2015, it was.
if(cal.get(Calendar.DAY_OF_WEEK) == Calendar.MONDAY){
System.out.println("This will print is my calendar secessfully gathers that monday was the first day of the month on June 2015.")
}
It doesn't execute.

Try this:
cal.set(Calendar.DATE, 1);
java.text.SimpleDateFormat sdf = new java.text.SimpleDateFormat("EEEE");
System.out.println(sdf.format(cal.getTime()));
Output:
Saturday
Demo

Java SE 8 has a whole new API for date and time, java.time. See Tutorial.
You can use the class LocalDate to get the day of a given week. For example, using your day from above do,
LocalDate d = LocalDate.of(2011, 10, 1);
Then LocalDate::getDayOfWeek() method will return a DayOfWeek enum instance, such as SATURDAY.
DayOfWeek dayOfWeek = localDate.getDayOfWeek ();
That enum can render a localized string for your sentence output.
String output = dayOfWeek.getDisplayName ( TextStyle.FULL , Locale.CANADA_FRENCH); // Or Locale.US or Locale.ENGLISH, and so on.
samedi

Android , Calendar.getInstance() not giving the correct month

I am trying to write code to find the Day difference between tow date but Calendar.getInstance() keep getting the date for previous month instead of current month
for example :Current 17/7/2014 it get 17/6/2014
my code :
TextView textview=(TextView) findViewById (R.id.textView1);
Calendar cal = Calendar.getInstance();
Calendar startDate=Calendar.getInstance();
startDate.set(Calendar.DAY_OF_MONTH, 1);
startDate.set(Calendar.MONTH,1);
startDate.set(Calendar.YEAR, 2013);
long diff=(((cal.getTimeInMillis()-startDate.getTimeInMillis())/(1000*60*60*24))+1);
String sdiff=String.valueOf(diff);
String stt=cal.get(Calendar.YEAR) +"_"+cal.get(Calendar.MONTH)+"_"+cal.get(Calendar.DAY_OF_MONTH);
textview.setText(stt);

Months start at 0, not at 1, but you really don't have to worry about this if you don't use magic numbers when getting or setting month but instead use the constants. So not this:
startDate.set(Calendar.MONTH,1); // this is February!
but rather
startDate.set(Calendar.MONTH, Calendar.JANUARY);

Months in Java's Calendar start with 0 for January, so July is 6, not 7.
Calendar.MONTH javadocs:
The first month of the year in the Gregorian and Julian calendars is JANUARY which is 0
Add 1 to the result of get.
(cal.get(Calendar.MONTH) + 1)
This also affects your set call. You can either subtract 1 when passing a month number going in, or you can use a Calendar constant, e.g. Calendar.JANUARY.
You can also use a SimpleDateFormat to convert it to your specific format, without having to worry about this quirk.
SimpleDateFormat sdf = new SimpleDateFormat("yyyy_MM_dd");
String stt = sdf.format(cal.getTime());

Calendar returning the wrong time

The time displayed is way ahead of what I expected. I'm parsing a date string and turning it into milliseconds.
year = Integer.parseInt(m1.group(1));
mo = Integer.parseInt(m1.group(2));
day = Integer.parseInt(m1.group(3));
hr = Integer.parseInt(m1.group(4));
min = Integer.parseInt(m1.group(5));
sec = Integer.parseInt(m1.group(6));
and here I set the Calendar
Calendar cal = Calendar.getInstance();
cal.set(year, mo, day, hr, min, sec);
time = cal.getTimeInMillis();

If you check out the calendar documentation here, then visit here, you'll see that January is month 0. You'll want to change your code to mo = Integer.parseInt(m1.group(2))-1;

You should probably use DateFormatter to parse the date string (rather than rolling your own).
Other than that, make sure that you have the proper time zone and understand that month number one is February (not January).