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claculate the number of days between two dates without libraries

I need to claculate the number of days between two dates without using any date or calendar classes provided by any library.
Here's my idea:
numberOfDays = Math.abs((toYear - fromYear) * 365);
numberOfDays = numberOfDays + Math.abs((toMonth - fromMonth) * 12);
numberOfDays = numberOfDays + Math.abs((toDay - fromDay));
Thoughts?

How many days between the start date and the end of the month?
How many days in each full month until the end of the year?
How many days in each full year until the year of the end date (counting leap years)?
How many days in each full month until the last month?
How many days from the start of the last month until the end date?
Some of these numbers may be zero.

In Java 8 you can do the following:
long days = ChronoUnit.DAYS.between(LocalDate.of(2014, Month.MARCH, 01), LocalDate.of(2014, Month.FEBRUARY, 15));

Would something like this do?
//get 2 random dates
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date date1 = new Date();
Date date2 = sdf.parse("2014-9-12");
final long msInADay = 24*60*60*1000; //turn a day to ms
//divide time difference by the ms of a day
int difference = (int)((date2.getTime() - date1.getTime()) / msInADay);
System.out.println(Math.abs(difference));//Math.abs so you can subtract dates in any order.
EDIT after updating your question:
You can do this:
static int calcDayDiff(int startY, int startM, int startD, int endY, int endM, int endD){
int result = (startY - endY) * 365;
result += (startM - endM) * 31;
result += (startD - endD);
return Math.abs(result);
}
Testing with: System.out.println(calcDayDiff(2014,9,13,2013,8,12)); will print 397
Note though that this is not a very good solution since not every month contains 31 days and not every year 365. You can fix the month day difference by adding some simple logic inside the method to not always multiply by 31. Since it's an assignment i guess you will be ok to consider every year having 365 days.

public class test {
public static void main(String[] args) {
Calendar cal = Calendar.getInstance();
cal.set(2014, 8, 1, 0, 0, 0);
Date s = cal.getTime();
Date e = new Date();
System.out.println(days(s,e));
}
public static int days(Date start, Date end){
double aTms = Math.floor(start.getTime() - end.getTime());
return (int) (aTms/(24*60*+60*1000));
}
}

Finding every first Sunday of the month between 1900 and a user-input year

This is homework.
I am trying to find every occurrence of a Sunday landing on the first of a month between Jan 1 1900 (which we are assuming was a Monday) and Dec 31 of a year that the user inputs. The calendar extension is off-limits.
I am returning dates in the correct format, but they do not match up with the example code our instructor provided.
In the given example, an input of 1902 should return:
1 Apr 1900
1 Jul 1900
1 Sep 1901
1 Dec 1901
1 Jun 1902
For 1902, my code returns:
1 Mar 1900
1 Jan 1901
1 Apr 1901
1 May 1901
1 Feb 1902
1 Jun 1902
1 Jul 1902
import java.util.Scanner;
public class Sundays {
public static void main(String[] args) {
Scanner reader = new Scanner(System.in);
System.out.print("Enter the ending year: ");
int userInputYear = reader.nextInt();
int[] orderedLengthOfMonthsArray = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
String[] orderedNamesOfMonthsArray = {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
int month = 0;
int dayOfWeek = 1; // initialized to MONDAY Jan 1, 1900 -- Sunday would be #7
int dayOfMonth = 1;
for (int year = 1900; year <= userInputYear; year++) {
for (month = 0; month < orderedLengthOfMonthsArray.length; month++) {
for (dayOfMonth = 1; dayOfMonth <= orderedLengthOfMonthsArray[month]; dayOfMonth++) {
dayOfWeek++;
if (dayOfMonth == 1 && dayOfWeek == 7) {
System.out.println(dayOfMonth + " " + orderedNamesOfMonthsArray[month] + " " + year);
}
if (dayOfWeek == 8) {
dayOfWeek = 1;
}
}
}
}
}
}

Swap the if statement and the increase of dayOfWeek.
for (dayOfMonth = 1; dayOfMonth <= orderedLengthOfMonthsArray[month]; dayOfMonth++) {
if (dayOfMonth == 1 && dayOfWeek == 7) {
System.out.println(dayOfMonth + " " + orderedNamesOfMonthsArray[month] + " " + year);
}
dayOfWeek++;
if (dayOfWeek == 8) {
dayOfWeek = 1;
}
}
When you are in the dayOfMonth for loop, you already have the correct day of the week (initially Monday, Jan 1st, 1900), so if you first increase it, then the check afterwards would be incorrect.

if (dayOfWeek == 7) {
dayOfWeek = 1;
}
So your week has 6 days? I think you should either reset to zero or reset when dayOfWeek is 8.

Someone mentioned Joda-Time magic… So here's my solution using Joda-Time 2.3.
While the question (a homework assignment) forbade the use of added libraries…
The comparison may be enlightening.
The student should soon learn that the java.util.Date & Calendar classes are notoriously troublesome and should be avoided for real-world work. They are supplanted in Java 8 by the new java.time.* package which is inspired by Joda-Time and defined by JSR 310.
My example code uses LocalDate which should only be used if your are absolutely certain that you don't care about (a) time of day, and (b) time zones. I do not usually recommend this approach as naïve programmers who believe they need neither time nor zone often turn out to be mistaken.
// Start
LocalDate start = new LocalDate( 1900, 1, 1 );
// Stop
// Using "half-open" comparison. We care about December 31 of specified year, but we will test for January 1 of year after.
String input = "1930";
int year = Integer.valueOf( input );
year = ( year + 1 ); // Add one to get to next year, for "half-open" approach.
LocalDate stop = new LocalDate( year, 1, 1 );
// Collect each LocalDate where the first of the month is a Sunday.
java.util.List<LocalDate> localDates = new java.util.ArrayList<LocalDate>();
LocalDate localDate = start;
do {
if ( localDate.getDayOfWeek() == DateTimeConstants.SUNDAY ) { // Comparing 'int' primitive values.
localDates.add( localDate ); // Collect this LocalDate instance.
}
localDate = localDate.plusMonths( 1 ); // Move on to next month. Joda-Time is smart about various month-ends and leap-year.
} while ( localDate.isBefore( stop ) ); // "Half-Open" means test "<" rather than "<=".
Dump to console…
System.out.println( "The First-Of-The-Month days that are Sundays from " + start + " (inclusive) to " + stop + " (exclusive):" );
System.out.println( localDates );
The First-Of-The-Month days that are Sundays from 1900-01-01 (inclusive) to 1931-01-01 (exclusive):
[1900-04-01, 1900-07-01, 1901-09-01, 1901-12-01, 1902-06-01, 1903-02-01, 1903-03-01, 1903-11-01, 1904-05-01, 1905-01-01, 1905-10-01, 1906-04-01, 1906-07-01, 1907-09-01, 1907-12-01, 1908-03-01, 1908-11-01, 1909-08-01, 1910-05-01, 1911-01-01, 1911-10-01, 1912-09-01, 1912-12-01, 1913-06-01, 1914-02-01, 1914-03-01, 1914-11-01, 1915-08-01, 1916-10-01, 1917-04-01, 1917-07-01, 1918-09-01, 1918-12-01, 1919-06-01, 1920-02-01, 1920-08-01, 1921-05-01, 1922-01-01, 1922-10-01, 1923-04-01, 1923-07-01, 1924-06-01, 1925-02-01, 1925-03-01, 1925-11-01, 1926-08-01, 1927-05-01, 1928-01-01, 1928-04-01, 1928-07-01, 1929-09-01, 1929-12-01, 1930-06-01]

Just for fun, while you have explicitly mentioned not to use Calendar, here is how we could do it:
public static List<Date> mondaysFirst(int firstYear, int lastYear) {
final List<Date> dates = new ArrayList<>();
final Calendar c1 = Calendar.getInstance(Locale.US);
c1.set(firstYear, 0, 1, 0, 0, 0);
final Calendar c2 = Calendar.getInstance(Locale.US);
c2.set(lastYear, 11, 31, 23, 59, 59);
while (c1.before(c2)) {
final int dayOfTheWeek = c1.get(Calendar.DAY_OF_WEEK);
// is sunday
if (dayOfTheWeek == 1) {
dates.add(c1.getTime());
}
c1.add(Calendar.MONTH, 1);
}
return dates;
}
And to print the results:
final List<Date> dates = mondaysFirst(1900, 1902);
final SimpleDateFormat sf = new SimpleDateFormat("dd MMM yyyy");
for (Date date: dates) {
System.out.println(sf.format(date));
}
Working Example.
I'm sure that there is some Joda-Time magic to make this even shorter.

Calculating days between certain dates with certain days being excluded using java Calendar

I'm trying to come up with a way to calculate the number of days between two different dates, however there will be certain days of the week that are only to be accounted for. For example, let's say we want to calculate the number of work days between 8/1 and 8/31, but employee only works Monday, Tuesday and Wednesday. The result would be that this employee only works 15 days during that period.
Has anyone put together something similar using the java Calendar class?

Try Joda Time, is the best solution to manage Date and Time.
The code of dogbane corrected:
final Calendar current = Calendar.getInstance();
current.set(2011, 7, 1);
final Calendar end = Calendar.getInstance();
end.set(2011, 7, 31);
int count = 0;
while (!current.after(end)) {
int dayOfWeek = current.get(Calendar.DAY_OF_WEEK);
if (dayOfWeek == Calendar.MONDAY ||
dayOfWeek == Calendar.TUESDAY ||
dayOfWeek == Calendar.WEDNESDAY) {
count++;
}
current.add(Calendar.DAY_OF_MONTH, 1);
}
System.out.println(count);
If use Joda Time:
DateTime current = new DateTime(2011, 8, 1, 0, 0, 0, 0);
DateTime end = new DateTime(2011, 8, 31, 0, 0, 0, 0);
int count = 0;
while (!current.isAfter(end)) {
int dayOfWeek = current.getDayOfWeek();
if (dayOfWeek == DateTimeConstants.MONDAY || dayOfWeek == DateTimeConstants.TUESDAY
|| dayOfWeek == DateTimeConstants.WEDNESDAY) {
count++;
}
current = current.plusDays(1);
}
System.out.println(count);

You can do this by incrementing the date by one day until you reach the end date. At each iteration, check if the day is Mon, Tue or Wed and increment a counter.
For example:
final Calendar current = Calendar.getInstance();
current.set(2011, 7, 1);
final Calendar end = Calendar.getInstance();
end.set(2011, 7, 31);
int count = 0;
while (current.compareTo(end) != 0) {
current.add(Calendar.DAY_OF_MONTH, 1);
int dayOfWeek = current.get(Calendar.DAY_OF_WEEK);
if (dayOfWeek == Calendar.MONDAY ||
dayOfWeek == Calendar.TUESDAY ||
dayOfWeek == Calendar.WEDNESDAY) {
count++;
}
}
System.out.println(count);

You could roll your own solution with java.util.Calendar; however, I suggest looking at some existing library, such as ObjectLab Kit date utilities.

How to calculate "Friday before" in Java or Groovy?

I need to be able to calculate the "Friday before" today in Java or Groovy.
For example, if today is Monday, February 21, the "Friday before" would be Friday, February 18.
And if today was Tuesday, February 1, the "Friday before" would be Friday, January 28.
What would be the best way to do this? What existing classes can I most effectively leverage?

You can use a loop:
Calendar c = Calendar.getInstance();
while(c.get(Calendar.DAY_OF_WEEK) != Calendar.FRIDAY)
{
c.add(Calendar.DAY_OF_WEEK, -1)
}
Or
Calendar c = Calendar.getInstance();
c.add(Calendar.DAY_OF_WEEK, -((c.get(Calendar.DAY_OF_WEEK) + 1) % 7));

I would make a method that gave me the number of days that have passed since the given day.
// Uses now by default
public static int daysSince(int day) {
return daysSince(day, Calendar.getInstance());
}
// Gives you the number of days since the given day of the week from the given day.
public static int daysSince(int day, Calendar now) {
int today = now.get(Calendar.DAY_OF_WEEK);
int difference = today - day;
if(difference <= 0) difference += 7;
return difference;
}
// Simple use example
public static void callingMethod() {
int daysPassed = daysSince(Calendar.FRIDAY);
Calendar lastFriday = Calendar.getInstance().add(Calendar.DAY_OF_WEEK, -daysPassed);
}

Convert a Julian Date to Regular Calendar Date

How do I convert a 7-digit julian date into a format like MM/dd/yyy?

Found a useful site: http://www.rgagnon.com/javadetails/java-0506.html
This should do the trick:
public static int[] fromJulian(double injulian) {
int jalpha,ja,jb,jc,jd,je,year,month,day;
double julian = julian + HALFSECOND / 86400.0;
ja = (int) julian;
if (ja>= JGREG) {
jalpha = (int) (((ja - 1867216) - 0.25) / 36524.25);
ja = ja + 1 + jalpha - jalpha / 4;
}
jb = ja + 1524;
jc = (int) (6680.0 + ((jb - 2439870) - 122.1) / 365.25);
jd = 365 * jc + jc / 4;
je = (int) ((jb - jd) / 30.6001);
day = jb - jd - (int) (30.6001 * je);
month = je - 1;
if (month > 12) month = month - 12;
year = jc - 4715;
if (month > 2) year--;
if (year <= 0) year--;
return new int[] {year, month, day};
}

Starting with Java 8, this becomes a one-liner to get the LocalDate:
LocalDate.MIN.with(JulianFields.JULIAN_DAY, julianDay)
.format(DateTimeFormatter.ofPattern("MM/dd/yyyy"));
Where julianDay is your 7-digit number.

simple way is here and this will return approx 100% accurate information.
String getDobInfo(double doubleString){
SweDate sweDate = new SweDate(doubleString);
int year = sweDate.getYear();
int month = sweDate.getMonth();
int day = sweDate.getDay();
// getting hour,minute and sec from julian date
int hour = (int) Math.floor(sweDate.getHour());
int min = (int) Math
.round((sweDate.getHour() - Math.floor(hour)) * 60.0);
int sec = (int) (((sweDate.getHour() - Math.floor(hour)) * 60.0 - Math
.floor(min)) * 60.0);
return "DOB:(DD:MM:YY) "+day+":"+month+":"+year+" TOB:(HH:MM:SS) "+hour+":"+min+":"+sec;
}
download the Swiss Ephemeris library and enjoy coding!!!

Do you really mean a Julian date, like astronomers use? Ordinal dates, which are specified as a year (four digits) and the day within that year (3 digits), are sometimes incorrectly called Julian dates.
static String formatOrdinal(int year, int day) {
Calendar cal = Calendar.getInstance();
cal.clear();
cal.set(Calendar.YEAR, year);
cal.set(Calendar.DAY_OF_YEAR, day);
Date date = cal.getTime();
SimpleDateFormat formatter = new SimpleDateFormat("MM/dd/yyyy");
return formatter.format(date);
}
This will give you the date at 00:00 local time; you may want to set the timezone on the calendars to GMT instead, depending on the application.

I see there are enough answers already provided. But any calendar related question is only half answered without mentioning joda-time ;-). Here is how simple it is with this library
// setup date object for the Battle of Hastings in 1066
Chronology chrono = JulianChronology.getInstance();
DateTime dt = new DateTime(1066, 10, 14, 10, 0, 0, 0, chrono);