Convert interface to int64 does not work as expected

I am trying to write a method to convert epoch timestamps to int64 values, but the method may get multiple data types; e.g. int64, int, string. I have following piece of code for it:

package main

import (
    "fmt"
)

func test(t interface{}) {
    tInt64, ok := t.(int64)
    fmt.Println("initial value:", t)
    fmt.Printf("initial type: %T\n", t)
    fmt.Println("casting status:", ok)
    fmt.Println("converted:", tInt64)
}

func main() {
    t := 1606800000
    tStr := "1606800000"

    test(t)
    test(tStr)

}

I expect it to successfully convert both t and tStr variables to int64; but, here's the result:

initial value: 1606800000
initial type: int
casting status: false
converted: 0
initial value: 1606800000
initial type: string
casting status: false
converted: 0

I don't know if it's related or not; but I executed the code with three version of golang compiler: 1.13, 1.14 and 1.15. All had same output.