The function user_use_date_picker () is not part of core FA 2.3 but in includes/current_user.inc in FA 2.4. Assume it is part of your module.
function user_use_date_picker()
{
return $_SESSION["wa_current_user"]->prefs->use_date_picker();
}
The function get_js_date_picker is in FA 2.3 and FA 2.4 in includes/ui/ui_view.inc.
The version of the above function in FA 2.3 uses:
$fpath = company_path().'/js_cache/'.'date_picker.js';
whilst that in FA 2.4 uses:
$fpath = user_js_cache().'/'.'date_picker.js';