Is there any way to find the response time, when I have two dates?

Where are you wanting to do this comparison? In-process it would just be a matter of fetching the two dates and making the comparison. The best way to accomplish this in widget scripting would be to make a custom extension method to compare two dates and return the difference according to your specifications.

I wanted to do using widget script api, I have two datetime (eg : 30-09-2021 13:08:08) But I need to convert them into days format, so that I can substract both days, and get Reponse time in form No of days

Instead of doing this directly in Velocity, I recommend either making a custom extension method as described above, or using Server-side javascript (jsm file) to define an extension method to use in Velocity. 

For example, add this file 'api.jsm' to the widget in question:

return {
    dateDiff: function(endDate, startDate) {
        // Calculate number of days between two dates
        return Math.floor((endDate - startDate) / (1000*60*60*24));
    }
};

Then reference it in your widget like so:

#set($api = $core_v2_widget.ExecuteFile('api.jsm'))

<span>
$api.dateDiff($firstCommentDate, $blogCreatedDate) days until first reply
</span>

Instead of doing this directly in Velocity, I recommend either making a custom extension method as described above, or using Server-side javascript (jsm file) to define an extension method to use in Velocity. 

For example, add this file 'api.jsm' to the widget in question:

return {
    dateDiff: function(endDate, startDate) {
        // Calculate number of days between two dates
        return Math.floor((endDate - startDate) / (1000*60*60*24));
    }
};

Then reference it in your widget like so:

#set($api = $core_v2_widget.ExecuteFile('api.jsm'))

<span>
$api.dateDiff($firstCommentDate, $blogCreatedDate) days until first reply
</span>

Thank you so much, It's Working

HTH! Note that if you want to use this in multiple widgets, you'll have to copy this to each one, whereas a custom extension would be available to all widgets automatically.