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		// Step 2: Generalise it
		// Trim off the timezone to replace at the end
		$output = $exemplarTimestamp;
		$tzRegex = '/\s*\(\w+\)\s*$/';
		$tzMatches = array();
		preg_match( $tzRegex, $output, $tzMatches );
		$output = preg_replace( $tzRegex, '', $output );
		$output = preg_quote( $output, '/' );
		$output = preg_replace( '/[^\d\W]+/u', '[^\d\W]+', $output );
		$output = preg_replace( '/\d+/u', '\d+', $output );

		$output .= preg_quote( $tzMatches[0] );


I guess it means the regex didn't match...

Change 173506 had a related patch set uploaded by Reedy:
Fix Undefined index: 0

https://gerrit.wikimedia.org/r/173506