TOPIC:

I testing and working tcalendar i need change date to day before current date 
 DTPModifiedDate.DateTime:= Date-1;

but still show me the current date selected not the previos day or day before to day..

how i can change date....   

DTPModifiedDate.DateTime is TDateTime

You must use
procedure DecodeDate
function TryEncodeDate

from datih.inc

function DateTimeToTimeStamp(DateTime: TDateTime): TTimeStamp;
function TimeStampToDateTime(const TimeStamp: TTimeStamp): TDateTime;
function MSecsToTimeStamp(MSecs: Comp): TTimeStamp;
function MSecsToTimeStamp(MSecs: Int64): TTimeStamp;
function TimeStampToMSecs(const TimeStamp: TTimeStamp): Int64;
function TryEncodeDate(Year, Month, Day: Word; out Date: TDateTime): Boolean;
function TryEncodeTime(Hour, Min, Sec, MSec: Word; out Time: TDateTime): Boolean;
function EncodeDate(Year, Month, Day :word): TDateTime;
function EncodeTime(Hour, Minute, Second, MilliSecond:word): TDateTime;
function ComposeDateTime(Date,Time : TDateTime) : TDateTime;
procedure DecodeDate(Date: TDateTime; out Year, Month, Day: word);
function DecodeDateFully(const DateTime: TDateTime; out Year, Month, Day, DOW: Word): Boolean;    

from sysutils.pp

OR to use 
 DTPModifiedDate.Date: String;