SystemReader: Offer methods with java.time API

Error prone explains: The Date API is full of major design flaws and
pitfalls and should be avoided at all costs. Prefer the java.time
APIs, specifically, java.time.Instant (for physical time) and
java.time.LocalDate[Time] (for civil time).

Add to SystemReader methods to get the time and timezone in the new
java.time classes (Instant/ZoneId) and mark as deprecated their old
counterparts.

The mapping of methods is:
* #getCurrentTime -> #now (returns Instant instead of int)
* #getTimezone -> #getTimeZoneAt (returns ZoneOffset intead of int)
* #getTimeZone -> #getTimeZoneId (return ZoneId instead of TimeZone)

Change-Id: Ic55b2f442a40046ff0ed24f61f566fc7416471be

org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java

@@ -18,6 +18,9 @@
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
 import java.time.Duration;
+import java.time.Instant;
+import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
@@ -201,6 +204,11 @@ public long getCurrentTime() {
 		return now;
 	}
 
+	@Override
+	public Instant now() {
+		return Instant.ofEpochMilli(now);
+	}
+
 	@Override
 	public MonotonicClock getClock() {
 		return () -> {
@@ -236,11 +244,21 @@ public int getTimezone(long when) {
 		return getTimeZone().getOffset(when) / (60 * 1000);
 	}
 
+	@Override
+	public ZoneOffset getTimeZoneAt(Instant when) {
+		return getTimeZoneId().getRules().getOffset(when);
+	}
+
 	@Override
 	public TimeZone getTimeZone() {
 		return TimeZone.getTimeZone("GMT-03:30");
 	}
 
+	@Override
+	public ZoneId getTimeZoneId() {
+		return ZoneOffset.ofHoursMinutes(-3, 30);
+	}
+
 	@Override
 	public Locale getLocale() {
 		return Locale.US;

org.eclipse.jgit/.settings/.api_filters

@@ -36,4 +36,18 @@
             </message_arguments>
         </filter>
     </resource>
+    <resource path="src/org/eclipse/jgit/util/SystemReader.java" type="org.eclipse.jgit.util.SystemReader">
+        <filter id="336695337">
+            <message_arguments>
+                <message_argument value="org.eclipse.jgit.util.SystemReader"/>
+                <message_argument value="getTimeZoneAt(Instant)"/>
+            </message_arguments>
+        </filter>
+        <filter id="336695337">
+            <message_arguments>
+                <message_argument value="org.eclipse.jgit.util.SystemReader"/>
+                <message_argument value="now()"/>
+            </message_arguments>
+        </filter>
+    </resource>
 </component>

org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java

@@ -25,6 +25,9 @@
 import java.nio.file.Paths;
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
+import java.time.Instant;
+import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.util.Locale;
 import java.util.TimeZone;
 import java.util.concurrent.atomic.AtomicReference;
@@ -166,10 +169,20 @@ public long getCurrentTime() {
 			return System.currentTimeMillis();
 		}
 
+		@Override
+		public Instant now() {
+			return Instant.now();
+		}
+
 		@Override
 		public int getTimezone(long when) {
 			return getTimeZone().getOffset(when) / (60 * 1000);
 		}
+
+		@Override
+		public ZoneOffset getTimeZoneAt(Instant when) {
+			return getTimeZoneId().getRules().getOffset(when);
+		}
 	}
 
 	/**
@@ -227,10 +240,20 @@ public long getCurrentTime() {
 			return delegate.getCurrentTime();
 		}
 
+		@Override
+		public Instant now() {
+			return delegate.now();
+		}
+
 		@Override
 		public int getTimezone(long when) {
 			return delegate.getTimezone(when);
 		}
+
+		@Override
+		public ZoneOffset getTimeZoneAt(Instant when) {
+			return delegate.getTimeZoneAt(when);
+		}
 	}
 
 	private static volatile SystemReader INSTANCE = DEFAULT;
@@ -501,9 +524,21 @@ private void updateAll(Config config)
 	 * Get the current system time
 	 *
 	 * @return the current system time
+	 *
+	 * @deprecated Use {@link #now()}
 	 */
+	@Deprecated
 	public abstract long getCurrentTime();
 
+	/**
+	 * Get the current system time
+	 *
+	 * @return the current system time
+	 *
+	 * @since 7.1
+	 */
+	public abstract Instant now();
+
 	/**
 	 * Get clock instance preferred by this system.
 	 *
@@ -520,19 +555,45 @@ public MonotonicClock getClock() {
 	 * @param when
 	 *            a system timestamp
 	 * @return the local time zone
+	 *
+	 * @deprecated Use {@link #getTimeZoneAt(Instant)} instead.
 	 */
+	@Deprecated
 	public abstract int getTimezone(long when);
 
+	/**
+	 * Get the local time zone offset at "when" time
+	 *
+	 * @param when
+	 *            a system timestamp
+	 * @return the local time zone
+	 * @since 7.1
+	 */
+	public abstract ZoneOffset getTimeZoneAt(Instant when);
+
 	/**
 	 * Get system time zone, possibly mocked for testing
 	 *
 	 * @return system time zone, possibly mocked for testing
 	 * @since 1.2
+	 *
+	 * @deprecated Use {@link #getTimeZoneId()}
 	 */
+	@Deprecated
 	public TimeZone getTimeZone() {
 		return TimeZone.getDefault();
 	}
 
+	/**
+	 * Get system time zone, possibly mocked for testing
+	 *
+	 * @return system time zone, possibly mocked for testing
+	 * @since 7.1
+	 */
+	public ZoneId getTimeZoneId() {
+		return ZoneId.systemDefault();
+	}
+
 	/**
 	 * Get the locale to use
 	 *