Introduce more extension methods

pgpainless-core/src/main/kotlin/openpgp/DateExtensions.kt

@@ -0,0 +1,23 @@
+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package openpgp
+
+import java.util.*
+
+    /**
+     * Return a new date which represents this date plus the given amount of seconds added.
+     *
+     * 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'
+     */
+    fun Date.plusSeconds(seconds: Long): Date? {
+        require(Long.MAX_VALUE - time > seconds) { "Adding $seconds seconds to this date would cause time to overflow." }
+        return if (seconds == 0L) null
+        else Date(this.time + 1000 * seconds)
+    }