some tests passed in clock

rust/gigasecond/src/lib.rs

@@ -0,0 +1,7 @@
+use time::PrimitiveDateTime as DateTime;
+use time::Duration;
+
+// Returns a DateTime one billion seconds after start.
+pub fn after(start: DateTime) -> DateTime {
+    start + Duration::seconds(1_000_000_000)
+}