No problem. I was looking through the Volume Delta Grid indicator you can get from the Community, it actually does this calculation and also formats it to be hh:mm:ss. I will post the snippet here, but not sure if the formatting will be right. The only difference I see is that this code stores the lastTs d.timestamp() in a state property instead of calling history.prior().timestamp().
const last = d.isLast() ? new Date(d.timestamp()) : new Date(lastTs)
const next = d.isLast() ? new Date() : new Date(d.timestamp())
const diff = (next - last) / 1000
const drawSecs = Math.floor(diff % 60)
const drawMins = Math.floor(diff / 60) % 60
const drawHrs = Math.floor(diff / 60 / 60)
const timeStr = `${
drawHrs < 10 ? '0'+drawHrs : drawHrs
}:${
drawMins < 10 ? '0'+drawMins : drawMins
}:${
drawSecs < 10 ? '0'+drawSecs : drawSecs
}`