To set date and time format and 1st week day options please use user/update_locale:

svc=user/update_locale&params={"userId":<long>,
                               "locale":
                                         {
                                             "fd":<text>,
                                             "wd":<uint>
                                         }
}

{
   "locale": 
             {
                "fd":<text>,   /* date and time format */
                "wd":<uint>    /* 1st week day: 1 - Monday, 7 - Sunday */
             }
}