This bittersweet song was co-written by Australian singer-songwriter Sia Furler, whose credits include Rihanna's "Diamonds
," David Guetta's "Titanium
" and Katy Perry's "Chained to the Rhythm
." The Aussie songstress has also experienced her own solo success with such hit tunes as "Chandelier
" and "Cheap Thrills
"Any association with Sia is a complete honor. We asked her for a song because we were such big fans of 'Titanium,'" Debbie Harry told Billboard
magazine. "She was writing with Nick Valensi from the Strokes, they submitted a couple songs and [we picked] the one we liked, 'Best Day Ever.'"