If the interface is implemented correctly, ZF2EdgeTest now already writes a BT-17 incl Date

library/src/main/java/org/mustangproject/ZUGFeRD/IDatedReference.java

@@ -0,0 +1,10 @@
+package org.mustangproject.ZUGFeRD;
+
+import java.util.Date;
+
+public interface IDatedReference {
+	public String getID();
+	default Date getDate() {
+		return null;
+	}
+}

library/src/main/java/org/mustangproject/ZUGFeRD/IExportableTransaction.java

@@ -214,6 +214,15 @@ public interface IExportableTransaction {
 		return null;
 	}
 
+	/**
+	 * BT-17 tender or lot reference
+	 *
+	 * @return mandatory ID, optional Date
+	 */
+	default IDatedReference getTenderReference() {
+		return null;
+	}
+
 	/**
 	 * own name
 	 *

library/src/test/java/org/mustangproject/ZUGFeRD/ZF2EdgeTest.java

@@ -144,6 +144,25 @@ public class ZF2EdgeTest extends MustangReaderTestCase {
 		return "DE";
 	}
 
+	@Override
+	public IDatedReference getTenderReference() {
+		return new IDatedReference() {
+			@Override
+			public String getID() {
+				return "983-jk-787 (obsolete)";
+			}
+			@Override
+			public Date getDate() {
+				SimpleDateFormat sdf=new SimpleDateFormat("YYYY-mm-dd");
+				try {
+					return sdf.parse("2025-10-12");
+				} catch (ParseException e) {
+					// wont happen, I promise :-)
+				}
+			}
+		};
+	}
+
 	@Override
 	public String getOwnLocation() {
 		return "Stadthausen";