Program size problem on attiny85 and not functional working code

This code is to take date data from hc-06 and comparing it with current date. If it is out of date, ledpin should turned on, else turned off.

I have 2 specific problems on this code.

1-On arduino uno r3, this code was working with no problems. But on Attiny85, whatever bluetooth data it takes, led turns on.

2-Since Attiny85 allows only 6012 bayt, this program is 6436 bayt (107 %). Is there a way to decrease its size? Thanks for all answers.

#include <virtuabotixRTC.h> //Library used
#include <SoftSerial.h>

SoftSerial btSerial(0, 2); // RX, TX
byte ledpin = 1; 
char BluetoothData;
String data, result;
long mday=0, mmonth=0, myear=0;
bool isDataAvailable;


//Wiring SCLK -> 6, I/O -> 7, CE -> 8
//Or CLK -> 6 , DAT -> 7, Reset -> 8

virtuabotixRTC myRTC(3, 4, 5);

void setup() {
 Serial.begin(9600);
 btSerial.begin(9600);
 pinMode(ledpin,OUTPUT);

 // seconds, minutes, hours, day of the week, day of the month, month, year
 //myRTC.setDS1302Time(0, 29, 15, 2, 8, 4, 2019); //Here you write your actual time/date as shown above 
}

void loop() {
 myRTC.updateTime();

  if (btSerial.available()){
    delay(10);
    result="";
    BluetoothData = btSerial.read();
    data+=BluetoothData;
    isDataAvailable=false;
  }

   if(result==""){
      mday=0;
      mmonth=0;
      myear=0;
    }
  
  if(data.length()==10){
   result=data; 
  }
  
  if (result.length() == 10) {
    if(isDataAvailable==false){
    for(int i=0; i<10; i++){
      if(i>=0 && i<2){
      mday*=10;
      mday+=data[i] - '0';
      }
      if(i>2 && i<=4){
      mmonth*=10;
      mmonth+=data[i] - '0';
      }
      if(i>5 && i<=9){
      myear*=10;
      myear+=data[i] - '0';
      if(i==9){
        isDataAvailable=true;
      }
      }
    }
    }
    if((mday<myRTC.dayofmonth && mmonth==myRTC.month && myear==myRTC.year) || (mmonth<myRTC.month && myear==myRTC.year) || (myear<myRTC.year)){
      digitalWrite(ledpin, HIGH);
    }
    else{
      digitalWrite(ledpin, LOW);
    }
    data="";
  }
}

Does it get smaller if you get rid of the Strings?

I am not sure how to handle bluetooth data without string

I didn't write without string, I wrote without String.

Okey. How can I handle bluetoothdata without using String?

By using C null terminated strings

Here's the documentation about strings:
https://www.arduino.cc/reference/en/language/variables/data-types/string/
You may find these functions useful when working with strings:
http://www.cplusplus.com/reference/cstring/

For comparison, here's the documentation about String:
https://www.arduino.cc/reference/en/language/variables/data-types/stringobject/

Thanks for answers!