Implement challenge 24 and finish set 3.

src/utils.rs

@@ -0,0 +1,9 @@
+use std::time::{SystemTime, UNIX_EPOCH};
+
+pub fn unix_timestamp() -> u32 {
+    let start = SystemTime::now();
+    let since_the_epoch = start
+        .duration_since(UNIX_EPOCH)
+        .expect("Time went backwards");
+    since_the_epoch.as_secs() as u32
+}