I understand the type cast warning in gcc, although vala should avoid any such warnings in C. The runtime warning, however, is more problematic.

$ cat test.vala 
public class Test:GLib.Object {
        public Time date { get; set; }

	public static void main () {
		Test t = new Test ();

		t.date = 1;
	}
}
$
$ valac test.vala 
test.c: In function ‘test_main’:
test.c:24: warning: passing argument 2 of ‘test_set_date’ from incompatible pointer type
$ ./test 

(process:21691): GLib-GObject-WARNING **: IA__g_object_notify: object class `Test' has no property named `date'
$ 

If I add this attribute, [CCode (notify = false)], the runtime warning disappears. 

Concluding it's the same or a similar issue as with the Gtk.TreeIter. Marking this bug as depending on the bug about that.