Code for updating the date time value based on Time Zone in apex salesforce 

 public string getDateAsString(string originalDate,string Type)
 {
       
        String dateOutput='';
        system.debug('inside getDateAsString');
   
        if(originalDate!=null&&originalDate!='')
        {
            system.debug('**originalDate**'+originalDate);
            if(Type=='date')
            {
                Datetime Datevalue = datetime.valueOf(originalDate);
                dateOutput= Datevalue.format('MM/dd/yyyy');

            }
            else if(Type=='datetime')
            {
                Datetime Datevalue = datetime.valueOf( originalDate);
                TimeZone tz = UserInfo.getTimeZone();
                Double offsetDate=tz.getOffset(Datevalue)/3600000;
                Datevalue=Datevalue.addHours(Integer.valueOf(offsetDate));
                dateOutput= Datevalue.format('MM/dd/yyyy hh:mm:ss a');
            }
        }
         system.debug('**dateOutput**'+dateOutput);
         return dateOutput;
 }