If you're just interested in timestamps in GMT you can also do this, which can be conveniently adapted for different intervals (hour: 1000 * 60 * 60
, 12 hours: 1000 * 60 * 60 * 12
, etc.)
const interval = 1000 * 60 * 60 * 24; // 24 hours in milliseconds
let startOfDay = Math.floor(Date.now() / interval) * interval;
let endOfDay = startOfDay + interval - 1; // 23:59:59:9999