JSFiddle - React, Tailwind, and code Playground

JavaScript

var rock = {
 0: 0.8,
 2: 1.0,
 5: 0.5,
 10: 0.3
}

var given = 4.3;
var timeA = null;
var timeB = null;
var keys = Object.keys(rock);
var sorted = keys.map(Number).sort(function(a,b) {return a[0]-b[0];});
for(var i = 0; i < sorted.length; i++){
    var e = keys[i];
    if(e > given){
        timeB = e;
        break;
    }
    timeA = e;    
}
var valueA = rock[timeA];
var valueB = rock[timeB];
console.log(timeA + ":" + valueA);
console.log(timeB + ":" + valueB);