Give me the first day of next month
JavaScript (V8), 67, 57, 56 bytes

-10 thanks to Arnauld,
-1 joining template string and ternary

s=&gt;([a,b]=s.split`-`,+a+!(b%=12))+`-${b++&lt;9?'0'+b:b}-01`


Test cases:



f=s=&gt;([a,b]=s.split`-`,+a+!(b%=12))+`-${b++&lt;9?'0'+b:b}-01`

console.log(f('2018-12-20'))
console.log(f('2019-05-06'))