Prefer Date.getFullYear() (#920)

MDN says `.getYear` is deprecated.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getYear

modern/src/runtime/System.ts

@@ -449,7 +449,7 @@ class System extends MakiObject {
   // returns the datetime's year since 1900
   getdateyear(datetime: number): number {
     const date = new Date(datetime * 1000);
-    return date.getYear();
+    return date.getFullYear() - 1900;
   }
 
   // returns the datetime's month (0-11)