Bad conversion from EndOfTheMonth(date) to Variant value

Question

I have a TDateTime value (that I get as result from EndOfTheMonth(date)) to a variant type. The result is wrongly rounded. Let's have a look at example:

  data := EndOfTheMonth(date);
  V := data;
  ShowMessage(DateTimeToStr(data) + ' vs ' + VarToStr(V));
 // output is
 // data = 2012-01-31 23:59:59
 // v    = 2012-02-01            // why next day?

Is it designed behaviour? How to work around this?

Efferent

Answer 1

ShowMessage(DateTimeToStr(data) + ' vs ' + DateTimeToStr(VarToDateTime(V)));

Update: I would guess the problem is that the last millisecond of the month is very close to 0:00:00 the next day, that is, the TDateTime value (which is basically a double) is very close to an integer (e.g. 41029.9999999884 is very close to 41029) and so the VarToStr function assumes the decimals to be numerical fuzz.

Ferdie

DateTimeToStr(V) shows the correct result too. Apparently, the issue is with the VarToStr() function. – Proa

@AndriyM: True, but it is never (?) wrong to be too explicit! :) – Ferdie

@JustMe: I haven't studied that page very carefully, but I am pretty sure that the best workaround for this particular problem is the one I gave above (or the slightly briefer version suggested by Andrily M). – Ferdie

@AndreasRejbrand Thanks - they use vartostr() function to check if TDateTime values are converted correctly to Variants, and assumed that way that this is not a bug in their software? – Efferent

I guess not. I was only adding more evidence to point out that the issue is not related to rounding (when assigning a TDateTime to a Variant). After digging a bit more, I found out that VarToStr() does nothing criminal: just calls VarToStrDef(), which in turn merely performs this assignment: Result := V (i.e. (string) := (Variant)). So, the conversion to string is done at a lower level, I guess. (Note: I used Delphi 6 in my research.) – Proa

@AndriyM: I saw that too. But it still appears to be some kind of numerical inaccuracy here (call it 'rounding' or not). For instance, if you IncHour(EndOfTheMonth(date), -2) instead of EndOfTheMonth(date) then you get 2012-04-30 21:59:59 vs 2012-04-30 22:00:00 if you use VarToStr. – Ferdie

Ah yes, of course it must have something to do with rounding, just not at the time of assignment, but rather at the time of string conversion. – Proa