Question : For Experts: Internal representation of pre-20th century dates... what do you think of this one?

Hello all,

This one is new to me.

I remember having a problem with expressions like CDate(-0.5), but I never explored the matter properly. So here is a table with dates, sorted ascending, with the numerical representation in the second column.

TheDate (sorted)          Value
----------------------------------------
27.12.1899 00:00:00      -3.00
28.12.1899 18:00:00      -2.75
28.12.1899 12:00:00      -2.50
28.12.1899 06:00:00      -2.25
28.12.1899 00:00:00      -2.00
29.12.1899 18:00:00      -1.75
29.12.1899 12:00:00      -1.50
29.12.1899 06:00:00      -1.25
29.12.1899 00:00:00      -1.00
30.12.1899 18:00:00      -0.75
30.12.1899 12:00:00      -0.50
30.12.1899 06:00:00      -0.25
30.12.1899 00:00:00       0.00
30.12.1899 06:00:00       0.25
30.12.1899 12:00:00       0.50
30.12.1899 18:00:00       0.75
31.12.1899 00:00:00       1.00
31.12.1899 06:00:00       1.25
31.12.1899 12:00:00       1.50
31.12.1899 18:00:00       1.75
01.01.1900 00:00:00       2.00
02.01.1900 00:00:00       3.00

Notice anything strange?

Apparently, pre-20th century date/time fields do not sort property, and, perhaps worse, there are two distinct representations for the same date/time on day zero!

    ? CDate(0.5) = CDate(-0.5)
    False
    ? Format(0.5, "ddddd ttttt") = Format(-0.5, "ddddd ttttt")
    True
    ? DateAdd("h", 0, -0.5) = DateAdd("h", 0, 0.5)
    True

Wow!

Let's hope that DateAdd and DateDiff work despite this. I guess Access never uses the range ]-1;0[ because:

    ? DateDiff("h", -1.25, 0.25)
     24
    ? DateDiff("h", -1.25, -0.25)
     23
    ? DateDiff("h", -0.25, 0.25)
     1

Ouch!

Of course I know the story behind the choice of day 1 for MS dates. The Excel team liked the Sunday 1st of January 1900 as Day 1, forgetting that 1900 isn't a leap year after all (the 29th of February is still a valid date in Excel!). So naturally Day 1 is formatted in Access as Sunday 31st of December 1899, which is correct.

However, the interpretation of the decimal part as time is just plain @€$#§!

Any thoughts, comments or reactions? For serious M$ flames, I should perhaps open a companion Lounge question, but please keep it civil here...

Markus
(°v°)

Answer : For Experts: Internal representation of pre-20th century dates... what do you think of this one?

This is one of my favorites.

Your findings are correct, and the mess - in addition to the wonderful story behind date value zero - is caused by the fact that the construction of date-time values in VB(A) with and integer and a decimal part leaves "a hole" in a way that can be compared to the missing year 0 in hour calender (remember: the year before year 1 is year -1).

Here are two solutions to the sorting issue:

As you have demonstrated, old dates - those with negative value, prior to 1899-12-30 - do not sort correctly if they contain a time part.
You may never need it but if you do, the usual and simple method is to use Format:

  SortDate = Format([DateField], "yyyymmddhhnnss")

and sort on that, but it is slow.
Another and faster method is this:

  SELECT
    *
  FROM
    tblDates
  ORDER BY
    Fix([DateField]),
    Abs([DateField]);

Fix takes care of keeping the dates in correct sequence (Int can not be used!) while Abs for a given date allows to sort the timepart correctly.

/gustav