Remove "seconds_since_midnight" from epi/eframe. Use chrono instead

chrono works both natively and on web.

Related: https://github.com/emilk/egui/issues/212

egui_demo_lib/src/lib.rs

@@ -154,3 +154,19 @@ fn test_egui_zero_window_size() {
         assert!(clipped_meshes.is_empty(), "There should be nothing to show");
     }
 }
+
+// ----------------------------------------------------------------------------
+
+/// Time of day as seconds since midnight. Used for clock in demo app.
+pub(crate) fn seconds_since_midnight() -> Option<f64> {
+    #[cfg(feature = "chrono")]
+    {
+        use chrono::Timelike;
+        let time = chrono::Local::now().time();
+        let seconds_since_midnight =
+            time.num_seconds_from_midnight() as f64 + 1e-9 * (time.nanosecond() as f64);
+        Some(seconds_since_midnight)
+    }
+    #[cfg(not(feature = "chrono"))]
+    None
+}