Crear tu cuenta atrás en JavaScript
javaScript countdown/count-up code (copy and paste to your Web page):

TargetDate = "12/31/2020 5:00 AM";BackColor = "palegreen";ForeColor = "navy";CountActive = true;CountStepper = -1;LeadingZero = true;DisplayFormat = "%%D%% Days, %%H%% Hours, %%M%% Minutes, %%S%% Seconds.";FinishMessage = "It is finally here!";

//countdown.js file//

/*Author: Robert Hashemianhttp://www.hashemian.com/

You can use this code in any manner so long as the author'sname, Web address and this disclaimer is kept intact.********************************************************Usage Sample:

TargetDate = "12/31/2020 5:00 AM";BackColor = "palegreen";ForeColor = "navy";CountActive = true;CountStepper = -1;LeadingZero = true;DisplayFormat = "%%D%% Days, %%H%% Hours, %%M%% Minutes, %%S%% Seconds.";FinishMessage = "It is finally here!";

*/

function calcage(secs, num1, num2) {  s = ((Math.floor(secs/num1))%num2).toString();  if (LeadingZero && s.length < 2)    s = "0" + s;  return "" + s + "";}

function CountBack(secs) {  if (secs < 0) {    document.getElementById("cntdwn").innerHTML = FinishMessage;    return;  }  DisplayStr = DisplayFormat.replace(/%%D%%/g, calcage(secs,86400,100000));  DisplayStr = DisplayStr.replace(/%%H%%/g, calcage(secs,3600,24));  DisplayStr = DisplayStr.replace(/%%M%%/g, calcage(secs,60,60));  DisplayStr = DisplayStr.replace(/%%S%%/g, calcage(secs,1,60));

  document.getElementById("cntdwn").innerHTML = DisplayStr;  if (CountActive)    setTimeout("CountBack(" + (secs+CountStepper) + ")", SetTimeOutPeriod);}

function putspan(backcolor, forecolor) { document.write("");}

if (typeof(BackColor)=="undefined")  BackColor = "white";if (typeof(ForeColor)=="undefined")  ForeColor= "black";if (typeof(TargetDate)=="undefined")  TargetDate = "12/31/2020 5:00 AM";if (typeof(DisplayFormat)=="undefined")  DisplayFormat = "%%D%% Dias, %%H%%:%%M%%,%%S%%";if (typeof(CountActive)=="undefined")  CountActive = true;if (typeof(FinishMessage)=="undefined")  FinishMessage = "";if (typeof(CountStepper)!="number")  CountStepper = -1;if (typeof(LeadingZero)=="undefined")  LeadingZero = true;

CountStepper = Math.ceil(CountStepper);if (CountStepper == 0)  CountActive = false;var SetTimeOutPeriod = (Math.abs(CountStepper)-1)*1000 + 990;putspan(BackColor, ForeColor);var dthen = new Date(TargetDate);var dnow = new Date();if(CountStepper>0)  ddiff = new Date(dnow-dthen);else  ddiff = new Date(dthen-dnow);gsecs = Math.floor(ddiff.valueOf()/1000);CountBack(gsecs);