TRange<T> = record
  private
    FMin, FMax: T;
  public
    function Contains(Value: T): Boolean;
  end;
...
function TRange<T>.Contains(Value: T): Boolean;
begin
 Result := (Value >= FMin) and (Value <= FMax) //E2015 Operator not applicable to this operand type
end;

  TRange = record
  private
    FMin, FMax: Variant;
  public
    constructor Create(MinValue, MaxValue: Variant);
    function Contains(Value: Variant): Boolean;
    function Random: Variant;
  end;

function TRange.Contains(Value: Variant): Boolean;
begin
 Result := False;
 case VarType(Value) of
  varSmallInt,varInteger,varShortInt..varUInt64,varSingle..varCurrency:
    Result := (Value >= FMin) and (Value <= FMax);
  varDate: if Value < 1
    then Result := (Int(FMin) + Value >= FMin) and (Int(FMin) + Value <= FMax)
    else Result := (Value >= FMin) and (Value <= FMax);
 end;
end;

constructor TRange.Create(MinValue, MaxValue: Variant);
begin
 FMin := MinValue;
 FMax := MaxValue;
end;

function TRange.Random: Variant;
begin
 Result := 0;
 case VarType(FMin) of
  varSmallInt,varInteger,varShortInt..varUInt64:
    Result := FMin + System.Random(FMax);
  varSingle..varCurrency:
    Result := FMin + System.Random * (FMax - FMin);
  varDate: if FMin < 1
    then Result := FMin + System.Random * (FMax - FMin)
    else if Frac(FMin) = 0
         then Result := FMin + Int(System.Random * (FMax - FMin))
         else Result := FMin + System.Random * (FMax - FMin);
 end;
end;

 Memo1.Lines.Add(TRange.Create(1, 100).Random);
 Memo1.Lines.Add(TRange.Create(0.1, 9.1).Random);
 Memo1.Lines.Add(TRange.Create(StrToDate('01.01.2021'), StrToDate('10.01.2021')).Random);
 Memo1.Lines.Add(TRange.Create(StrToTime('00:00'), StrToTime('23:59')).Random);
 Memo1.Lines.Add(TRange.Create(StrToDateTime('01.01.2021 00:01'), StrToDateTime('10.01.2021 23:59')).Random);