Date routines ("High Level"):
  Overview        About these routines
  XPboM()         Return Beginning-of-Month                                  .
  XPeoM()         Return End-of-Month                                        .
  XPboY()         Return beginning-of-Year                                   .
  XPeoY()         Return End-of-Year                                         .
  XPprevMonth()   Return same date in previous Month                         .
  XPnextMonth()   Return same date in next Month                             .
  XPprevYear()    Return same date in previous Year                          .
  XPnextYear()    Return same date in next Year                              .
  XPmonthLen()    Return length of month                                     .
  XPyearLen()     Return length of Year                                      .
  XPleapYear()    Return whether year is a Leap year                         .
  XPweekNr()      Return Week Number according to ISO standards              .
  XPweekYear()    Return Year applying to Number return by WeekNr            .
  XPnullDate()    Return a NULL date                                         .
  XPdoy()         Return day of Year                                         .