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

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?

Solution

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.