The Show Numeric Day of Week routine returns the numeric day of
    the week for an input time value. If 0 is the input time value,
    the current day of the week is returned. The days are numbered 1
    through 7, with Monday as day 1 and Sunday as day 7.

    Format

      LIB$DAY_OF_WEEK  [user-time,] day-number

1  –  Returns

    OpenVMS usage:cond_value
    type:         longword (unsigned)
    access:       write only
    mechanism:    by value

2  –  Arguments

 user-time

    OpenVMS usage:date_time
    type:         quadword (unsigned)
    access:       read only
    mechanism:    by reference

    Time to be translated to a day of the week, or zero. The
    optional user-time argument is the address of an unsigned
    quadword containing the value of time. Time must be supplied
    as an absolute system time. To obtain this time value in proper
    quadword format, call the $BINTIM system service.

    If time is passed as zero by value, the numeric value for
    the current day is returned. If time is passed as a zero by
    reference, the number returned represents the day of November
    17, 1858. If the user-time argument is omitted, it is equivalent
    to passing a zero by value.

 day-number

    OpenVMS usage:longword_unsigned
    type:         longword (unsigned)
    access:       write only
    mechanism:    by reference

    Numeric day of week. The day-number argument is the address of
    a longword into which LIB$DAY_OF_WEEK writes the integer value
    representing the day of the week.