Two answers required. In 1999, Martin achieved his first Billboard #1 as a songwriter, a mark he would hit not one, but twenty-one more times up through his most recent in 2016 — quite the incessant sensation! What TWO SONGS bookend this feat? (I will also accept the artist's name for either one.)
...BABY ONE MORE TIME (Britney Spears); CAN'T STOP THE FEELING! (Justin Timberlake)