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WordPress supports external feeds, and parse the feed using a function named fetch_feed() from SimplePie. RSS widget is one of them using the same function to fetch external feed.

The Codex says:

fetch_feed caches results for 12 hours by default.

So, the feed you are fetching is actually at least 12 hours old. But a good news is there also that, you can modify the duration of the fetching by using a filter: wp_feed_cache_transient_lifetime.

How to use that? Simple. Just paste the following code into your theme's functions.php:

add_filter( 'wp_feed_cache_transient_lifetime', function(){ return 1800; });

Here,
we are using the filter to modify the duration. Here we are passing our new duration with 1800 [seconds]. The calculation is: 30 minutes = 30 min. x 60 sec. = 1800 sec.

Using the simple code you can change the duration.

Source:

• RSS Widget doesn’t update - Internet Builders
• Fix the RSS update time on the WordPress RSS widget - village/idiot.org