The “Now onwards” string is translated with _x() which is different than __() because it allows context to be added to the string being translated.
Because there is context, you need to use gettext_with_context instead of gettext. Here’s a simple example based on the one in the WP.org Code Reference:
function example_gettext_with_context( $translated, $text ) {
if ( 'Now' == $text ) {
$translated = 'Today';
}
return $translated;
}
add_filter( 'gettext_with_context', 'example_gettext_with_context', 10, 2 );
You can find the relavent code in the Events Calendar plugin here and here
The Events Calendar website also has some detailed examples.