How to correctly get date and time passed between two dates?

Question

I am creating this function:

function LiteralTimePassed(FromDate: TDateTime; ToDate: TDateTime = 0): string;
const
  Coma = ', ';
var
  Dt, Mt: Integer; { se dos dias contarem mais que 30/31 então aumenta o mês }
  P: TDateTime;
  HC: Boolean; { indica se já há um token antes do novo token, para colocar vírgula }
  Token: string; { a parte do timestamp verificada no loop }
  Literal: string;
  Y, M, D, H, N, S: Integer; { ano, mês, dia, hora, minuto(n), segundo }
  Ys, Ms, Ds, Hs, Ns, Ss: Boolean; { valida se valores maiores que 1 para adicionar 's' }
begin
{ retorna quanto tempo se passou desde a data 1 até a data 2 em forma literal }
  if ToDate = 0 then ToDate := Now;
  HC := False;
  Literal := '';
  P := ToDate-FromDate ;
   Dt := (DaysInMonth(FromDate)-DayOf(FromDate))+(DaysInMonth(ToDate)-DayOf(ToDate));
   Mt := Dt div DaysInMonth(ToDate);
  Ys := VarAssign(Y, YearsBetween(ToDate, FromDate)) > 1;
  Ms := VarAssign(M, (MonthsBetween(ToDate, FromDate)-(Y*MonthsPerYear))-Mt) > 1;
  Ds := VarAssign(D, Dt mod DaysInMonth(ToDate)) > 1;
  // I did not make the hour, minute and second, yet...
end;

To get a response like:

 There has been "2 years, 4 months, 1 day, 7 hours, 30 minutes and 22 seconds" between those dates

My doubt is if my logic is correct in the case of counting days passed and if I will have to make the same calculations with the time part.

But if you know any free coded that already does that, it will save me much time!

Thanks.

Answer 1

First deal with years and months, after that TTimeSpan:

procedure TimePassed(dt1,dt2: TDateTime);
var
  y1,m1,d1,h1,mi1,s1,ms,y2,m2,d2,h2,mi2,s2,y,mo,d:word;
  ts:TTimeSpan;
begin
  DecodeDateTime(dt1,y1,m1,d1,h1,mi1,s1,ms);
  DecodeDateTime(dt2,y2,m2,d2,h2,mi2,s2,ms);
  ms:=12*y2+m2-12*y1-m1;
  if s1+60*mi1+60*60*h1+24*60*60*d1>s2+60*mi2+60*60*h2+24*60*60*d2 then ms:=ms-1;
  mo:= ms mod 12;
  y:=ms div 12; //years and months ready, now the rest
  dt1:=EncodeDateTime(y1+y,m1+mo,d1,h1,mi1,s1,0);
  ts := TTimeSpan.Subtract(dt2, dt1);
  Result:= Format('There has been "%d years, %d months, %d days, %d hours, %d minutes and %d seconds" between those dates',
    [y,mo,ts.Days, ts.Hours, ts.Minutes, ts.Seconds]);
end;