ZmanimFormatter.formatXSDDurationTime: write negative durations as valid xsd:duration

Duration.toString() writes a negative duration as "PT-1H-15M-3S", which isn't
a valid xsd:duration (XSD only allows one leading sign: "-PT1H15M3S"). This
also affected format() with XSD_DURATION_FORMAT and the durations in toXML()
/ toJSON().

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Er4Sx7LnKzzCbgDEatxkU

Author: Elyahu41

src/main/java/com/kosherjava/zmanim/util/ZmanimFormatter.java

@@ -282,8 +282,8 @@ public String formatXSDateTime(Instant instant) {
 	}
 
 	/**
-	 * This returns the xml representation of an xsd:duration object. This simply uses {@link Duration#toString()} that formats it
-	 * in the standard XSD ISO-8601 (e.g., "{@code PT1H30M}")
+	 * This returns the xml representation of an xsd:duration object, in the standard XSD ISO-8601 format (e.g., "{@code PT1H30M}",
+	 * or "{@code -PT1H30M}" for a negative duration).
 	 * 
 	 * @param duration the duration to format
 	 * @return the xsd:duration ISO-8601 formatted String
@@ -292,6 +292,11 @@ public String formatXSDDurationTime(Duration duration) {
 		if (duration == null) {
 			return "";
 		}
+		if (duration.isNegative()) {
+			// Duration.toString() writes a negative duration with a sign on every field ("PT-1H-30M"), which isn't a valid
+			// xsd:duration. XSD puts a single sign in front of the whole value.
+			return "-" + duration.negated().toString();
+		}
 		return duration.toString(); // Java's native toString() outputs a standard XSD ISO-8601 (e.g., "PT1H30M")
 	}
 

src/test/java/com/kosherjava/zmanim/util/ZmanimFormatterXsdDurationTest.java

@@ -0,0 +1,57 @@
+/*
+ * Zmanim Java API
+ * Copyright © 2004-2026 Eliyahu Hershfeld
+ *
+ * This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General
+ * Public License as published by the Free Software Foundation; version 2.1 of the License.
+ *
+ * This library is distributed in the hope that it will be useful,but WITHOUT ANY WARRANTY; without even the implied
+ * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more
+ * details.
+ * You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA,
+ * or connect to: https://www.gnu.org/licenses/old-licenses/lgpl-2.1.html
+ */
+package com.kosherjava.zmanim.util;
+
+import static org.junit.Assert.assertEquals;
+
+import java.time.Duration;
+import java.time.ZoneId;
+
+import javax.xml.datatype.DatatypeFactory;
+
+import org.junit.Test;
+
+/**
+ * {@link ZmanimFormatter#formatXSDDurationTime(Duration)} must produce a valid {@code xsd:duration}, including for negative
+ * durations (for example a negative offset, or a <em>sha'ah zmanis</em> computed from a start after the end).
+ */
+public class ZmanimFormatterXsdDurationTest {
+	private final ZmanimFormatter formatter = new ZmanimFormatter(ZoneId.of("America/New_York"));
+
+	@Test
+	public void positiveDurationsAreUnchanged() {
+		assertEquals("PT1H30M", formatter.formatXSDDurationTime(Duration.ofMinutes(90)));
+		assertEquals("PT0S", formatter.formatXSDDurationTime(Duration.ZERO));
+		assertEquals("", formatter.formatXSDDurationTime(null));
+	}
+
+	@Test
+	public void negativeDurationsHaveALeadingSign() throws Exception {
+		assertEquals("-PT1H15M3S", formatter.formatXSDDurationTime(Duration.ofMinutes(-75).minusSeconds(3)));
+		assertEquals("-PT0.5S", formatter.formatXSDDurationTime(Duration.ofMillis(-500)));
+
+		// and the result is a valid xsd:duration with the same value
+		Duration negative = Duration.ofHours(-1).minusMinutes(15).minusMillis(250);
+		String text = formatter.formatXSDDurationTime(negative);
+		assertEquals(negative.toMillis(), DatatypeFactory.newInstance().newDuration(text).getTimeInMillis(new java.util.Date(0)));
+	}
+
+	@Test
+	public void formatUsesTheXsdFormatForNegativeDurations() {
+		ZmanimFormatter xsd = new ZmanimFormatter(ZmanimFormatter.XSD_DURATION_FORMAT,
+				java.time.format.DateTimeFormatter.ofPattern("h:mm:ss"), ZoneId.of("America/New_York"));
+		assertEquals("-PT30M", xsd.format(Duration.ofMinutes(-30)));
+	}
+}