updated dateinput.js to calculate proper top offset

public/javascripts/admin/dateinput.js

@@ -116,14 +116,28 @@ DateInputBehavior.Calendar = Behavior.create({
     this.date = this.selector.getDate();
     this.redraw();
     this.element.setStyle({
-      'top': this.selector.element.cumulativeOffset().top - (this.element.getHeight() + 4) + 'px',
+      'top': this.vertical_offset(this.selector.element) + 'px',
       'left': Math.max(this.selector.element.cumulativeOffset().left + this.selector.element.getWidth() - this.element.getWidth() - 4, this.selector.element.cumulativeOffset().left) + 'px',
       'z-index': 10001
     });
     this.element.show();
     this.active = true;
   },
 
+  vertical_offset: function(selector){
+    default_vertical_offset = this.selector.element.cumulativeOffset().top + this.selector.element.getHeight() + 2;
+    this_height = this.element.getHeight();
+    if(document.viewport.getHeight() > default_vertical_offset + this_height) {
+      top_value = default_vertical_offset;
+    } else {
+      top_value = (default_vertical_offset - this_height - selector.getHeight() - 6);
+    }
+    if(top_value < document.viewport.getScrollOffsets().top) {
+      top_value = document.viewport.getScrollOffsets().top;
+    }
+    return top_value;
+  },
+
   hide: function() {
     this.element.hide();
     this.active = false;