The Day Number Returned as a Longword Integer routine returns the
    number of days since the system zero date of November 17, 1858,
    or the number of days from November 17, 1858, to a user-supplied
    date.

    Format

      LIB$DAY  number-of-days [,user-time] [,day-time]

1  –  Returns

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

2  –  Arguments

 number-of-days

    OpenVMS usage:longword_signed
    type:         longword integer (signed)
    access:       write only
    mechanism:    by reference

    Number of days since the system zero date. The number-of-days
    argument is the address of a signed longword integer containing
    the day number.

 user-time

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

    User-supplied time, in 100-nanosecond units. The user-time
    argument is the address of a signed quadword integer containing
    the user time. A positive value indicates an absolute time, while
    a negative value indicates a delta time. This is an optional
    argument. If user-time is omitted, the default is the current
    system time. This quadword time value is obtained by calling 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, rather than the current day.

 day-time

    OpenVMS usage:longword_signed
    type:         longword integer (signed)
    access:       write only
    mechanism:    by reference

    Number of 10-millisecond units since midnight of the user-time
    argument. The day-time argument is the address of a signed
    longword integer into which LIB$DAY writes this number of units.