Date Difference in Months & Days

function TTDateDiffUtils.DateAddMonths(startDate: TDateTime; monthCount: integer; sticky: boolean): TDateTime;
var
  RY,RM,RD : word;  // result date
  SY,SM,SD : word;  // start date
begin
  result := IncMonth(startDate, monthCount);
  if (sticky) then // stick
  begin
    DecodeDate(result, RY,RM,RD);
    DecodeDate(startDate, SY,SM,SD);
    if DaysInAMonth(RY, RM) = SD then // i.e. if the day portion from the startDate === the end of the new month then make the result be the end of the month
      result := EncodeDate(RY, RM, SD);
  end;
end;

function TTDateDiffUtils.DateDiffString(aThen, aNow: TDateTime; sticky : boolean; out years, months, days, hours : integer ): string;
var
  LNow, LThen, LRunning: TDateTime;
  testDate : TDateTime;
  monthCount : integer;
begin

  //  https://secondboyet.com/Articles/PublishedArticles/Calculatingthenumberofmon.html  //

  result := '';

  if aThen > aNow then
  begin
    LNow := aThen;
    LThen := aNow;
  end else
  begin
    LNow := aNow;
    LThen := aThen;
  end;

  monthCount := Trunc((LNow-LThen) / 31);  // make a guess -- it'll be close but not 100% accurate
  testDate := DateAddMonths(LThen, monthCount, sticky);
  while (testDate < LNow) do
  begin
    Inc(monthCount);
    testDate := DateAddMonths(LThen, monthCount, sticky);
  end;

  if (testDate = LNow) then
  begin
    months := monthCount;
    days := 0;
  end else
  begin
    // testDate > LNow = we've exceeded the second date so backup there sparky and fix the values
    Dec(monthCount);
    testDate := DateAddMonths(LThen, monthCount, sticky);
    months := monthCount;
    days := Trunc((LNow-testDate));  // this is the remainder days
  end;

  result := format('%d months %d days', [months, days]);
end;