Power-Programmierung/CD1.mdf/pascal/library/dos/t_power/dates.doc

Text File | 1987-10-27 | 4.7 KB | 94 lines

  DATES  --  A unit providing Julian day numbers and date manipulations.

  Version 1.00 - 10/26/1987 - First general release

  Scott Bussinger
  Professional Practice Systems
  110 South 131st Street
  Tacoma, WA  98444
  [phone]
  Compuserve 72247,2671

  This UNIT for Turbo Pascal 4.0 provides a new data type Date, which is a
Julian day number indicating the number of days that have elapsed since
January 1, 1900.  A Date allows you to store any date from January 1, 1900 to
December 31, 2078 in only two bytes.  Also defined is an enumerated type
defining the days of the week.  The available routines support conversion
to/from a day, month and year format to the Date format, checking for valid
dates, adding a number of days, months and years to a Date, conversion to/from
a Date and a string form that can be used for sorting and a routine for
determining the day of the week for a given date.  To include this unit in
your program, add DATES to the USES clause in your main program.
  The advantages of using Julian Date numbers are the savings of much storage
space (2 bytes vs. 6 ASCII characters) and that the number of days between any
two dates is a simple subtraction.  The algorithms used here are adapted from
an article by Gordon King in the June 1983 issue of Dr. Dobb's Journal.  Mr.
King in turn credits Algorithm 199 in "The Collected Algorithms of the ACM"
(1963) by R. G. Tantzen.


type Date = Word;
    A date is stored as a word variable and represents the number of days
    elapsed since January 1, 1900.

type Day = (Sunday,Monday,Tuesday,Wednesday,Thursday,Friday,Saturday);
    A Day variable can take the value of any of these special constants
    representing the days of the week.

function ValidDate(Day,Month,Year: integer): boolean;
    This functions checks that the given date is both a valid date and that it
    is within the range of dates that can be stored in a Date variable.  Note
    that no error checking takes place in any of the other routines -- it is
    assumed that input error checking will call this routine and all
    parameters to the other routines will thus have alreay been checked.  Days
    range from 1..31 depending on the month and year, months range from 1..12,
    and years range from 1900..2078.

procedure DMYtoDate(Day,Month,Year: integer;var Julian: Date);
    Calculate the Julian day number for the given date and return as a Date.
    Again note that no error checking is taking place in this routine -- if
    the date given doesn't make sense then neither will the result, use
    ValidDate to check for valid dates first if necessary.

procedure DateToDMY(Julian: Date;var Day,Month,Year: integer);
    The reciprocal routine for DMYtoDate, this procedure takes a Julian day
    number and returns the day, month and year.

function BumpDate(Julian: Date;Days,Months,Years: integer): Date;
    This routine adds (or subtracts) a given number of days, months and years
    from the Date.  Months and years are added first before days are added.
    Note that no overflow or underflow checking is performed on the
    calculation, be sure that the resulting date will be in the valid range
    for a Date variable.

function DayOfWeek(Julian: Date): Day;
    This routine returns the day of the week for the given date.  This
    information can be used directly, or supplied to the DayString routine
    below to obtain a string representation of the day of the week.

function DayString(WeekDay: Day): string;
    This routine takes a Day value and converts it into a string with the name
    of the day for use in output.  The first letter of each is capitalized.

function MonthString(Month: integer): string;
    This routine takes an integer from 1..12 and converts it into a string
    with the name of the month for use in output.  The first letter of each is
    capitalized.

function DateToStr(Julian: Date): string;
    Returns a two character string representing the Date suitable for use
    in standard string sorts or comparisons.  For example, the result could
    be concatenated to a key for use with the Database Toolbox.

function StrToDate(StrVar: string): Date;
    The reciprocal function to DateToStr, this converts a sortable string back
    into a Date value.

const BlankDate = $FFFF;
    This special constant is not in the valid range for a Date variable and
    can be used to represent an non-entry, blank entry, or not-a-real-date in
    input and output routines.  Do not supply this value to any routines
    expecting a Date value.  If you decide to use this constant, it is
    expected that you will check for this special value before calling any of
    the above routines.