Im creating this email markup on Sendgrid and the design includes a countdown time showing how many day, hours,minutes and seconds there is before the deadline, havent tried using Javascript on sendgrid yet but is it possible? Do I just put a in there?
It looks like your question is rather "how can I do a countdown timer in email", and I'll answer that.
In a nutshell, you use animated gifs in combination with a server-side trick to change what image loads when the gifs display. That trick is to use a GIF-encoder that tricks the email client into thinking the php link is actually a GIF:
<img src="http://mosaichosting.co.uk/countdown/2/gif.php?time=2014-12-25+00:00:01" alt="Countdown" border="0" style="display:block;">
To setup the PHP script, follow these instructions (it's quite long): https://litmus.com/community/learning/27-how-to-add-a-countdown-timer-to-your-email
Or to have a setup do it for you there are free and paid alternatives. See this free one: https://www.sendtric.com/