since https://gerrit.wikimedia.org/r/#/c/149268/, there is a dropdown menu for selecting the max age of the changes shown in the watchlist. Additional to the seven options defined in SpecialWatchlist.php (1, 2, 6, 12 hours; 1, 3, 7 days), there is an option '30' which is the $wgRCMaxAge, but is not defined in cutoffselector function of SpecialWatchlist.php and therefore nonfunctional. If you select the '30' in the dropdown and click 'apply', '&days=' is simply set to zero.

I am no MediaWiki expert but I had a look in the SpecialWatchlist.php change and spotted the following:
Code before the change (cutoffLinks function)

global $wgRCMaxAge;
$watchlistMaxDays = ceil( $wgRCMaxAge / ( 3600 * 24 ) );

$hours = array( 1, 2, 6, 12 );
$days = array( 1, 3, 7, $watchlistMaxDays );

Code after the change (cutoffselector function):

$hours = array( 1, 2, 6, 12 );
$days = array( 1, 3, 7 );

Maybe the $watchlistMaxDays got lost mistakenly?