How do I work around Delphi's inability to accurately handle datetime manipulations?

Question

I am new to Delphi (been programming in it for about 6 months now). So far, it's been an extremely frustrating experience, most of it coming from how bad Delphi is at handling dates and times. Maybe I think it's bad because I don't know how to use TDate and TTime properly, I don't know. Here is what is happening on me right now :

// This shows 570, as expected
ShowMessage(IntToStr(MinutesBetween(StrToTime('8:00'), StrToTime('17:30'))));

// Here I would expect 630, but instead 629 is displayed. WTF!?
ShowMessage(IntToStr(MinutesBetween(StrToTime('7:00'), StrToTime('17:30'))));

That's not the exact code I use, everything is in variables and used in another context, but I think you can see the problem. Why is that calculation wrong? How am I suppose to work around this problem?

Answer 1

Given

a := StrToTime('7:00');
b := StrToTime('17:30');

ShowMessage(FloatToStr(a));
ShowMessage(FloatToStr(b));

your code, using MinutesBetween, effectively does this:

ShowMessage(IntToStr(trunc(MinuteSpan(a, b)))); // Gives 629

However, it might be better to round:

ShowMessage(IntToStr(round(MinuteSpan(a, b)))); // Gives 630

What is actually the floating-point value?

ShowMessage(FloatToStr(MinuteSpan(a, b))); // Gives 630

so you are clearly suffering from traditional floating-point problems here.

Update:

The major benefit of Round is that if the minute span is very close to an integer, then the rounded value will guaranteed be that integer, while the truncated value might very well be the preceding integer.

The major benefit of Trunc is that you might actually want this kind of logic: Indeed, if you turn 18 in five days, legally you are still not allowed to apply for a Swedish driving licence.

So you if you'd like to use Round instead of Trunc, you can just add

function MinutesBetween(const ANow, AThen: TDateTime): Int64;
begin
  Result := Round(MinuteSpan(ANow, AThen));
end;

to your unit. Then the identifier MinutesBetween will refer to this one, in the same unit, instead of the one in DateUtils. The general rule is that the compiler will use the function it found latest. So, for instance, if you'd put this function above in your own unit DateUtilsFix, then

implementation

uses DateUtils, DateUtilsFix

will use the new MinutesBetween, since DateUtilsFix occurss to the right of DateUtils.

Update 2:

Another plausible approach might be

function MinutesBetween(const ANow, AThen: TDateTime): Int64;
var
  spn: double;
begin
  spn := MinuteSpan(ANow, AThen);
  if SameValue(spn, round(spn)) then
    result := round(spn)
  else
    result := trunc(spn);
end;

This will return round(spn) is the span is within the fuzz range of an integer, and trunc(spn) otherwise.

For example, using this approach

07:00:00 and 07:00:58

will yield 0 minutes, just like the original trunc-based version, and just like the Swedish Trafikverket would like. But it will not suffer from the problem that triggered the OP's question.

Answer 2

This is an issue that is resolved in the latest versions of Delphi. So you could either upgrade, or simply use the new code in Delphi 2010. For example this program produces the output you expect:

{$APPTYPE CONSOLE}
uses
  SysUtils, DateUtils;

function DateTimeToMilliseconds(const ADateTime: TDateTime): Int64;
var
  LTimeStamp: TTimeStamp;
begin
  LTimeStamp := DateTimeToTimeStamp(ADateTime);
  Result := LTimeStamp.Date;
  Result := (Result * MSecsPerDay) + LTimeStamp.Time;
end;

function MinutesBetween(const ANow, AThen: TDateTime): Int64;
begin
  Result := Abs(DateTimeToMilliseconds(ANow) - DateTimeToMilliseconds(AThen))
    div (MSecsPerSec * SecsPerMin);
end;

begin
  Writeln(IntToStr(MinutesBetween(StrToTime('7:00'), StrToTime('17:30'))));
  Readln;
end.

---

The Delphi 2010 code for MinutesBetween looks like this:

function SpanOfNowAndThen(const ANow, AThen: TDateTime): TDateTime;
begin
  if ANow < AThen then
    Result := AThen - ANow
  else
    Result := ANow - AThen;
end;

function MinuteSpan(const ANow, AThen: TDateTime): Double;
begin
  Result := MinsPerDay * SpanOfNowAndThen(ANow, AThen);
end;

function MinutesBetween(const ANow, AThen: TDateTime): Int64;
begin
  Result := Trunc(MinuteSpan(ANow, AThen));
end;

So, MinutesBetween effectively boils down to a floating point subtraction of the two date/time values. Because of the inherent in-exactness of floating point arithmetic, this subtraction can yield a value that is slightly above or below the true value. When it is below the true value, the use of Trunc will take you all the way down to the previous minute. Simply replacing Trunc with Round would resolve the problem.

---

As it happens the latest Delphi versions, completely overhaul the date/time calculations. There are major changes in DateUtils. It's a little harder to analyse, but the new version relies on DateTimeToTimeStamp. That converts the time portion of the value to the number of milliseconds since midnight. And it does so like this:

function DateTimeToTimeStamp(DateTime: TDateTime): TTimeStamp;
var
  LTemp, LTemp2: Int64;
begin
  LTemp := Round(DateTime * FMSecsPerDay);
  LTemp2 := (LTemp div IMSecsPerDay);
  Result.Date := DateDelta + LTemp2;
  Result.Time := Abs(LTemp) mod IMSecsPerDay;
end;

Note the use of Round. The use of Round rather than Trunc is the reason why the latest Delphi code handles MinutesBetween in a robust fashion.

---

Assuming that you cannot upgrade right now, I would deal with the problem like this:

1. Leave your code unchanged. Continue to call MinutesBetween etc.
2. When you do upgrade, your code that calls MinutesBetween etc. will now work.
3. In the meantime fix MinutesBetween etc. with code hooks. When you do come to upgrade, you can simply remove the hooks.