Keep every Daf Yomi Yerushalmi daf and give the molad of Teves 3761

src/main/java/com/kosherjava/zmanim/hebrewcalendar/JewishDate.java

@@ -309,7 +309,7 @@ private static int getLastDayOfGregorianMonth(int year, int month) {
 	 * @return the {@code LocalDate}.
 	 */
 	private static LocalDate absDateToDate(int absDate) {
-	 	int year = absDate / 366; // Search forward year by year from approximate year
+	 	int year = Math.floorDiv(absDate, 366); // Search forward year by year from approximate year
 	 	while (absDate >= gregorianDateToAbsDate(year + 1, 1, 1)) {
 	 	 	year++;
 	 	}
@@ -347,11 +347,12 @@ private static int gregorianDateToAbsDate(int year, int month, int dayOfMonth) {
         }
 
         int y = year - 1;
+        // Floor division, so that year 0, a leap year, gets its leap day
 		return (absDate // days this year
 				+ 365 * y // days in previous years ignoring leap days
-                + y / 4 // Julian leap days before this year
-                - y / 100 // minus prior century years
-                + y / 400); // plus prior years divisible by 400
+                + Math.floorDiv(y, 4) // Julian leap days before this year
+                - Math.floorDiv(y, 100) // minus prior century years
+                + Math.floorDiv(y, 400)); // plus prior years divisible by 400
     }
 
     /**
@@ -670,13 +671,11 @@ public int getDaysInJewishMonth() {
     }
 
     /**
-     * Computes and sets the Jewish date fields based on the provided absolute (Gregorian) date.
+     * Computes and sets the Jewish date fields based on the provided absolute (Gregorian) date. The date may be before
+     * January 1, 0001, as the <em>molad</em> of Teves 3761 is, on December 12, 0000.
      * @param gregorianAbsDate the Gregorian absolute date.
      */
     private synchronized void setAbsDate(int gregorianAbsDate) {
-        if (gregorianAbsDate <= 0) {
-            throw new IllegalArgumentException("Dates in the BC era are not supported");
-        }
         this.gregorianAbsDate = gregorianAbsDate;
         // Approximation from below
         jewishYear = (gregorianAbsDate - JEWISH_EPOCH) / 366;
@@ -696,8 +695,8 @@ private synchronized void setAbsDate(int gregorianAbsDate) {
         // Calculate the day by subtraction
         jewishDay = gregorianAbsDate - jewishDateToAbsDate(jewishYear, jewishMonth, 1) + 1;
 
-        // day of week (same calculation as original)
-        dayOfWeek = Math.abs(gregorianAbsDate % 7) + 1;
+        // day of week, with floorMod so that a date before January 1, 0001 gets the right day
+        dayOfWeek = Math.floorMod(gregorianAbsDate, 7) + 1;
     }
 
     /**
@@ -855,6 +854,9 @@ public void setGregorianDate(ZonedDateTime zonedDateTime) {
      */
     public void setGregorianDate(LocalDate localDate) {
         int absDate = gregorianDateToAbsDate(localDate.getYear(),  localDate.getMonth().getValue(), localDate.getDayOfMonth());
+        if (absDate <= 0) {
+            throw new IllegalArgumentException("Dates in the BC era are not supported");
+        }
         setAbsDate(absDate); // convert to Jewish date
     }
 
@@ -975,7 +977,11 @@ public void minusDays(int days){
         if (days < 1) {
             throw new IllegalArgumentException("The number of days to subtract must be greater than zero.");
         }
-        setAbsDate(getAbsDate() - days);
+        int absDate = getAbsDate() - days;
+        if (absDate <= 0) {
+            throw new IllegalArgumentException("Dates in the BC era are not supported");
+        }
+        setAbsDate(absDate);
 
     }
 

src/main/java/com/kosherjava/zmanim/hebrewcalendar/YerushalmiYomiCalculator.java

@@ -149,14 +149,15 @@ private static int getNumOfSpecialDays(LocalDate start, LocalDate end) {
 
 
     /**
-     * Checks if a date is after start and on or before end.
+     * Checks if a date is on or after start and on or before end. A cycle can start on, or be extended to start
+     * on, Yom Kippur or Tisha B'Av, so the start day is counted.
      * @param start the start <code>ZonedDateTime</code>
      * @param date the <code>ZonedDateTime</code> to check
      * @param end the end <code>ZonedDateTime</code>
      * @return if the date is between the two dates
      */
     private static boolean isBetween(LocalDate start, LocalDate date, LocalDate end) {
-        return start.isBefore(date) && !end.isBefore(date);
+        return !date.isBefore(start) && !end.isBefore(date);
     }
 
     /**

src/test/java/com/kosherjava/zmanim/hebrewcalendar/UT_JewishDateNavigation.java

@@ -60,5 +60,18 @@ public void addYearsUsesConfiguredAdarMonthWhenMovingToLeapYear() {
 		Assert.assertEquals(10, adarBeisDate.getJewishDayOfMonth());
 	}
 
+	/**
+	 * The molad of Teves 3761 is Tuesday 12 December of Gregorian year 0, which is a leap year.
+	 */
+	@Test
+	public void moladInGregorianYear0() {
+		JewishDate molad = new JewishDate(3761, JewishDate.TEVES, 20).getMolad();
+		Assert.assertEquals(LocalDate.of(0, 12, 12), molad.getLocalDate());
+		Assert.assertEquals(Calendar.TUESDAY, molad.getDayOfWeek());
+		Assert.assertEquals(20, molad.getMoladHours());
+		Assert.assertEquals(7, molad.getMoladMinutes());
+		Assert.assertEquals(2, molad.getMoladChalakim());
+	}
+
 
 } // End of UT_JewishDateNavigation class

src/test/java/com/kosherjava/zmanim/hebrewcalendar/UT_YerushalmiTest.java

@@ -1,5 +1,6 @@
 package com.kosherjava.zmanim.hebrewcalendar;
 
+import java.time.LocalDate;
 import java.util.Calendar;
 
 import org.junit.*;
@@ -50,4 +51,33 @@ public void testCorrectSpecialDate() {
 		System.out.println(hdf.formatDafYomiYerushalmi(jewishCalendar.getDafYomiYerushalmi()));
 	}
 
+	/**
+	 * The cycle ending in 2172 is extended for Tisha B'Av 5932. It still has every daf, ending with Niddah 13 on
+	 * 7 August 2172.
+	 */
+	@Test
+	public void testCycleExtendedForTishaBav() {
+		assertDaf(LocalDate.of(2172, 8, 6), 38, 12);
+		assertDaf(LocalDate.of(2172, 8, 7), 38, 13);
+		assertDaf(LocalDate.of(2172, 8, 8), 0, 1);
+	}
+
+	/**
+	 * Tisha B'Av 6240 falls the day before the last daf of a cycle. The cycle still has every daf, ending with
+	 * Niddah 13 on 16 August 2480, and the next one starts with Berachos 1.
+	 */
+	@Test
+	public void testCycleWithTishaBavBeforeItsLastDaf() {
+		assertDaf(LocalDate.of(2480, 8, 14), 38, 12);
+		Assert.assertNull(new JewishCalendar(LocalDate.of(2480, 8, 15)).getDafYomiYerushalmi());
+		assertDaf(LocalDate.of(2480, 8, 16), 38, 13);
+		assertDaf(LocalDate.of(2480, 8, 17), 0, 1);
+	}
+
+	private static void assertDaf(LocalDate date, int masechtaNumber, int daf) {
+		Daf dafYomi = new JewishCalendar(date).getDafYomiYerushalmi();
+		Assert.assertEquals(date.toString(), masechtaNumber, dafYomi.getMasechtaNumber());
+		Assert.assertEquals(date.toString(), daf, dafYomi.getDaf());
+	}
+
 }