Counting down to a time? - delphi

Question

I'm trying to count down to a time of the day (24-hour clock format). This is my solution so far:
function TimeDiffStr(const s1, s2: string): string;
var
t1, t2: TDateTime;
secs: Int64;
begin
t1 := StrToDateTime(s1);
t2 := StrToDateTime(s2);
secs := SecondsBetween(t1, t2);
Result := Format('%2.2d:%2.2d:%2.2d', [secs div 3600, (secs div 60) mod 60, secs mod 60]);
end;
procedure TForm1.Timer1Timer(Sender: TObject);
var
TargetTime: TTime;
s: string;
begin
s := TimeDiffStr(TimeToStr(Now), TimeToStr(TargetTime));
end;
If Now is, for example, 15:35:02 and the target time is 21:44:59, the output is correct (06:09:57). However, if Now is 15:35:02 and the target time is 01:32:23, instead of counting down from 09:57:21, it will count upwards, because the function does not know that the target time is on a different day.
How can I work out the difference between two times when the times are on different days?

Answer 1

First off, there is no need to pass strings around. If you start with TTime and convert to TTime, then simply pass TTime around.
Second, since you are dealing with just time values, if the target time is meant to be on the next day, you need to add 24 hours so that you have a TDateTime that actually represents the next day.
Try this:
uses
..., DateUtils;
function TimeDiffStr(const t1, t2: TTime): string;
var
d1, d2: TDateTime;
secs: Int64;
begin
d1 := t1;
if t2 < t1 then
d2 := IncDay(t2) // or IncHour(t2, 24)
else
d2 := t2;
secs := SecondsBetween(d1, d2);
Result := Format('%2.2d:%2.2d:%2.2d', [secs div 3600, (secs div 60) mod 60, secs mod 60]);
end;
procedure TForm1.Timer1Timer(Sender: TObject);
var
TargetTime: TTime;
s: string;
begin
TargetTime := ...;
s := TimeDiffStr(Time(), TargetTime);
end;