Mark content in a view that is older than 30 days

Not sure if this is the best way to go about doing this, but you can achieve this like so:

1) Exclude all the fields in your view from display.
2) Edit the event date field in your view to use a plain formatter. We need the date string only, not the html element.
3) Add a 'Custom Text' field into your view. Here, we will render the event fields manually and add logic to differentiate the classes using twig templating.

Here's the code I used to introduce a new class of old-event into the div which renders the individual events.

{% set currentDate = "now"|date("U") %}
{% set dateField =  field_event_date|date("U") %}
{% set timeDifference = currentDate - dateField %}
{% set daysDifference = timeDifference/86400 %}
{% if daysDifference > 30 %}

<div class="event-card old-event">
{% else %}
<div class="event-card">
{% endif %}

{{ title }}
{{ field_event_date }}

</div>

Hey sidharth_soman! Thank you very much, this is exactly, what i needed.