Add date checker

CHANGELOG.md

@@ -12,6 +12,8 @@ We refer to [GitHub issues](https://github.com/JabRef/jabref/issues) by using `#
 
 ### Changed
 We added RFC, a new entry generator ID type. [#3971](https://github.com/JabRef/jabref/issues/3971)
+We added a validity check for dates in the `date` and `urldate` fields.
+
 ### Fixed
 We fixed an issue where the export to clipboard functionality could not be invoked [#3994](https://github.com/JabRef/jabref/issues/3994)
 

src/main/java/org/jabref/logic/integrity/DateChecker.java

@@ -0,0 +1,24 @@
+package org.jabref.logic.integrity;
+
+import java.util.Optional;
+
+import org.jabref.logic.l10n.Localization;
+import org.jabref.model.entry.Date;
+import org.jabref.model.strings.StringUtil;
+
+public class DateChecker implements ValueChecker {
+
+    @Override
+    public Optional<String> checkValue(String value) {
+        if (StringUtil.isBlank(value)) {
+            return Optional.empty();
+        }
+
+        Optional<Date> parsedDate = Date.parse(value);
+        if (!parsedDate.isPresent()) {
+            return Optional.of(Localization.lang("incorrect format"));
+        }
+
+        return Optional.empty();
+    }
+}

src/main/java/org/jabref/logic/integrity/FieldCheckers.java

@@ -50,6 +50,11 @@ private static Multimap<String, ValueChecker> getAllMap(BibDatabaseContext datab
         fieldCheckers.put(FieldName.YEAR, new YearChecker());
         fieldCheckers.put(FieldName.KEY, new ValidBibtexKeyChecker(enforceLegalKey));
 
+        if (databaseContext.isBiblatexMode()) {
+            fieldCheckers.put(FieldName.DATE, new DateChecker());
+            fieldCheckers.put(FieldName.URLDATE, new DateChecker());
+        }
+
         return fieldCheckers;
     }
 

src/test/java/org/jabref/logic/integrity/DateCheckerTest.java

@@ -0,0 +1,28 @@
+package org.jabref.logic.integrity;
+
+import java.util.Optional;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class DateCheckerTest {
+
+    private DateChecker checker;
+
+    @BeforeEach
+    void setUp() {
+        checker = new DateChecker();
+    }
+
+    @Test
+    void complainsAboutInvalidIsoLikeDate() {
+        assertEquals(Optional.of("incorrect format"), checker.checkValue("2018-04-21TZ"));
+    }
+
+    @Test
+    void acceptsValidIsoDate() {
+        assertEquals(Optional.empty(), checker.checkValue("2018-04-21"));
+    }
+}