How would I use my date of birth to calculate age, birthstone, and chinese zodiac sign in kotlin?

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Android

Kotlin

I am working on an app using kotlin in android studio. I have two activities. One activity accepts the age the the user inputs and then it takes me to another activity that would show the age, birthstone, and Chinese Zodiac sign.
What i am having trouble doing is finding how to do the age, birthstone and zodiac part. Can i get some guidance please?

What I have tried:

For the first activity i have

 <pre>
class MainActivity : AppCompatActivity() {


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

val sendDOB = findViewById<EditText>(R.id.InputDOB)
val sendbutton = findViewById<Button>(R.id.createBirthdayCardButton)



        sendbutton.setOnClickListener {
            val dob = sendDOB.text.toString()
            val intent = Intent(this, Birthdaycard::class.java)
            intent.putExtra("DOB", dob)
            startActivity(intent)
} 
    }

For the second activity i have

<pre>class Birthdaycard : AppCompatActivity() {

    companion object {
        const val NAME_EXTRA = "NAME"
        const val DOB_EXTRA = "DOB"
    }


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_birthdaycard)

        val dob = intent.getStringExtra(DOB_EXTRA)

        val dobTextView = findViewById<TextView>(R.id.outputage)
        dobTextView.text = dob

        // calculate and display age, birthstone, and zodiac sign here




        }
    }