diff --git a/pgpainless-core/src/main/kotlin/openpgp/DateExtensions.kt b/pgpainless-core/src/main/kotlin/openpgp/DateExtensions.kt
index 2763cb55..0dc6de40 100644
--- a/pgpainless-core/src/main/kotlin/openpgp/DateExtensions.kt
+++ b/pgpainless-core/src/main/kotlin/openpgp/DateExtensions.kt
@@ -14,7 +14,6 @@ import java.util.*
  * Since '0' is a special date value in the OpenPGP specification (e.g. '0' means no expiration for
  * expiration dates), this method will return 'null' if seconds is 0.
  *
- * @param date date
  * @param seconds number of seconds to be added
  * @return date plus seconds or null if seconds is '0'
  */
@@ -25,9 +24,19 @@ fun Date.plusSeconds(seconds: Long): Date? {
     return if (seconds == 0L) null else Date(this.time + 1000 * seconds)
 }
 
+/** This date in seconds since epoch. */
 val Date.asSeconds: Long
     get() = time / 1000
 
+/**
+ * Return the number of seconds that would need to be added to this date in order to reach the later
+ * date. Note: This method requires the [later] date to be indeed greater or equal to this date,
+ * otherwise an [IllegalArgumentException] is thrown.
+ *
+ * @param later later date
+ * @return difference between this and [later] in seconds
+ * @throws IllegalArgumentException if later is not greater or equal to this date
+ */
 fun Date.secondsTill(later: Date): Long {
     require(this <= later) { "Timestamp MUST be before the later timestamp." }
     return later.asSeconds - this.asSeconds
diff --git a/pgpainless-core/src/test/java/openpgp/DateExtensionTest.java b/pgpainless-core/src/test/java/openpgp/DateExtensionTest.java
new file mode 100644
index 00000000..c36315bb
--- /dev/null
+++ b/pgpainless-core/src/test/java/openpgp/DateExtensionTest.java
@@ -0,0 +1,44 @@
+// SPDX-FileCopyrightText: 2025 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package openpgp;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Date;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class DateExtensionTest {
+
+    @Test
+    public void testDatePlusSecondsBaseCase() {
+        Date t0 = DateExtensionsKt.parseUTC("2025-05-12 10:36:53 UTC");
+        Date t1 = DateExtensionsKt.plusSeconds(t0, 7);
+        assertEquals("2025-05-12 10:37:00 UTC", DateExtensionsKt.formatUTC(t1));
+    }
+
+    @Test
+    public void testDatePlusZeroReturnsNull() {
+        Date t0 = DateExtensionsKt.parseUTC("2025-05-12 10:36:53 UTC");
+        Date t1 = DateExtensionsKt.plusSeconds(t0, 0);
+        assertNull(t1);
+    }
+
+    @Test
+    public void testDatePlusSecondsOverflowing() {
+        Date now = new Date();
+        // expect IAE because of time field overflowing
+        assertThrows(IllegalArgumentException.class, () ->
+                DateExtensionsKt.plusSeconds(now, Long.MAX_VALUE - 10000));
+    }
+
+    @Test
+    public void testParsingMalformedUTCTimestampThrows() {
+        assertThrows(IllegalArgumentException.class, () ->
+                DateExtensionsKt.parseUTC("2025-05-12 10:36:XX UTC"));
+    }
+}