Add generation timestamp to metadata (#516)

* Add generation timestamp to metadata

Co-authored-by: Daniel Beck <daniel-beck@users.noreply.github.com>

src/main/java/io/jenkins/update_center/json/WithSignature.java

@@ -14,18 +14,32 @@
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.security.GeneralSecurityException;
+import java.time.Instant;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeFormatter;
 
 /**
  * Support generation of JSON output with included checksum + signatures block for the same JSON output.
  */
 public abstract class WithSignature {
     private JsonSignature signature;
+    private final String generationTimestamp = DateTimeFormatter.ISO_DATE_TIME.format(Instant.now().atOffset(ZoneOffset.UTC).withNano(0));
 
     @JSONField
     public JsonSignature getSignature() {
         return signature;
     }
 
+    /**
+     * Returns a string with the current date and time in ISO-8601 format.
+     * It doesn't have fractional seconds and the timezone is always UTC ('Z').
+     *
+     * @return a string with the current date and time in the format YYYY-MM-DD'T'HH:mm:ss'Z'
+     */
+    public String getGenerationTimestamp() {
+        return generationTimestamp;
+    }
+
     /**
      * Generate JSON checksums and add a signature block to the JSON written to the specified {@link Writer}.
      *

src/test/java/io/jenkins/update_center/TimestampTest.java

@@ -0,0 +1,29 @@
+package io.jenkins.update_center;
+
+import io.jenkins.update_center.json.WithSignature;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.time.Instant;
+import java.time.temporal.ChronoUnit;
+
+public class TimestampTest {
+    @Test
+    public void checkTimestamp() throws Exception {
+        WithSignature w = new WithSignature() {
+        };
+
+        String timestamp = w.getGenerationTimestamp();
+
+        Assert.assertTrue("format as expected", timestamp.matches("^202[0-9][-][0-9]{2}[-][0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}Z$"));
+
+        Instant parsed = Instant.parse(timestamp);
+
+        Assert.assertTrue("before now", parsed.isBefore(Instant.now()));
+        Assert.assertTrue("very recent", parsed.isAfter(Instant.now().minus(1, ChronoUnit.SECONDS)));
+
+        String timestamp2 = w.getGenerationTimestamp();
+        Assert.assertEquals("Doesn't change over time", timestamp, timestamp2);
+
+    }
+}