Использовать TTime для более 24 часов

function TextToTime(S: string): Integer;
 var
   p, i: Integer;
   Sh, Sm, Ss: string;
 begin
   Sh := '';
   SM := '';
   SS := '';
   i  := 1;
   p  := 0;
   while i do
   begin
     if (s[i] <> ':') then
     begin
       case P of
         0: SH := Sh + s[i];
         1: SM := SM + S[i];
         2: SS := SS + S[i];
       end;
     end
     else
       Inc(p);
     Inc(i);
   end;
   try
     Result := (StrToInt(SH) * 3600) + (StrToInt(SM) * 60) + (StrToInt(SS))
     except
       Result := 0;
   end;
 end;
 
 function TimeToText(T: Integer): string;
 var
   H, M, S: string;
   ZH, ZM, ZS: Integer;
 begin
   ZH := T div 3600;
   ZM := T div 60 - ZH * 60;
   ZS := T - (ZH * 3600 + ZM * 60);
   if ZH then H := '0' + IntToStr(ZH)
   else
     H := IntToStr(ZH);
   if ZM then M := '0' + IntToStr(ZM)
   else
     M := IntToStr(ZM);
   if ZS then S := '0' + IntToStr(ZS)
   else
     S := IntToStr(ZS);
   Result := H + ':' + M + ':' + S;
 end;

Взято с сайта: https://www.swissdelphicenter.ch