SRT/VTT Subtitle Reverser

.js

// SRT or VTT string is stored in variable 's'.
var time=27*60000+11463 // Time of video in milliseconds.
var mult=5444/5444 // Timestamp multiplier AFTER 'time' offset.
var index=0
var isVtt

function f(v,i,a){
	if(v=='WEBVTT'){
		isVtt=true
		return''
	}
	var t=v.split('\n')
	if(isVtt)t.unshift(null)
	t[0]=++index
	//console.log(t)
	t[1]=t[1].split(' --> ').map(v=>{
		if(v.length==9)v='00:'+v
		var h=Number.parseInt(v.substring(0,2))
		var m=Number.parseInt(v.substring(3,5))
		var s=Number.parseInt(v.substring(6,8))
		var l=Number.parseInt(v.substring(9))
		var r=mult*(time-(1000*(60*(60*h+m)+s)+l))
		var H=Math.floor(r/3600000)
		var M=Math.floor(r/60000%60)
		var S=Math.floor(r/1000%60)
		var L=Math.floor(r%1000)
		if(H<10)H='0'+H
		if(M<10)M='0'+M
		if(S<10)S='0'+S
		if(L<10)L='00'+L
		else if(L<100)L='0'+L
		return ''+H+':'+M+':'+S+','+L
	}).reverse().join(' --> ')
	return t.join('\n')
}

var t=s.trim().split('\n\n')
var result=t.reverse().filter(s=>s.length).map(f).join('\n\n')
console.log(result)