timer in kivy of minute and second

Question

I am trying to create a timer in kivy for a label but whenever executing the code, getting the error TypeError: unsupported operand type(s) for -=: 'str' and 'int' here is the code for the same main file:

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.lang import Builder
from kivy.properties import ObjectProperty


Builder.load_file('pythonitcal CSS.kv')

class MyLayout(Widget):
    
    def start_timer(self):
        startTime=self.ids.timeInput.text
        # self.ids.time.text = startTime
        t=startTime
        t-=1
        i=59
        while t>=0:
            while i>=0:
                self.ids.time.text = t
                i-=1
            t-=1
            i=59

class Labels(App):
    def build(self):
        return MyLayout()


if __name__ == '__main__':
    Labels().run()

kivy file:

#:kivy 2.1.0
#:import utils kivy.utils
<Label>
    size_hint: (0.2,0.2)
    font_size: 32
    canvas.before:
        Color:
            rgba:utils.get_color_from_hex('#ef944a')
        Rectangle:
            pos: self.pos
            size: self.size
<MyLayout>
    
    canvas.before:
        Color:
            rgba:utils.get_color_from_hex('#429682')
        Rectangle:
            size: self.size
            pos: self.pos
    

    BoxLayout:
        orientation:'vertical'
        size:root.width,root.height
        padding: 15
        Label:
            id:time
            text:'Time left: '
            size_hint:(1,0.2)
        
        TextInput:
            id:timeInput
            multiline: False
        
        Button:
            text:'Start'
            on_press: root.start_timer()

you can add animation in the label part if it is needed, or even simple-looking timer can do the trick. thank you in advance