Create a custom javascript variable (i.e. a variable that contains a function, not a "javascript" variable that just reads a global variable), and give it a name, e.g. "timestamp".
Custom javascript variables are anonymous functions with a return value.
The current way to get a timestamp is Date.now().  This might not be supported by older browser (especially IE 8 and lower), so you might use new Date().getTime(); as an alternative. 
The variable body would be as simple as:
function() {
  return Date.now();
}
and you would use that in a tag by surrounding the variable name with double curly parenthesis, e.g. {{timestamp}}. Date.now() returns milliseconds ( elapsed since 1 January 1970 00:00:00 UTC), so you might want to divide by thousand.
Alternatively you could create a datetime variable that includes seconds and even milliseconds. I think this was originally by Simo Ahava:
function() {
    // Get local time as ISO string with offset at the end
    var now = new Date();
    var tzo = -now.getTimezoneOffset();
    var dif = tzo >= 0 ? '+' : '-';
    var pad = function(num) {
        var norm = Math.abs(Math.floor(num));
        return (norm < 10 ? '0' : '') + norm;
    };
    return now.getFullYear() 
        + '-' + pad(now.getMonth()+1)
        + '-' + pad(now.getDate())
        + 'T' + pad(now.getHours())
        + ':' + pad(now.getMinutes()) 
        + ':' + pad(now.getSeconds())
        + '.' + pad(now.getMilliseconds())
        + dif + pad(tzo / 60) 
        + ':' + pad(tzo % 60);
}
which returns a formatted string like 2016-08-02T09:22:44.496+02:00.