Give me the first day of next month

Question

Simple question!

Given a string that contains a date in ISO8601 format, print the first of the next month in ISO8601 format.

Example inputs:

2018-05-06

2019-12-20

Their respective outputs:

2018-06-01

2020-01-01

The rules:

• Imports are free and don't count towards your score.
• The date will be stored in a string called s.
• The solution with the least characters, wins

Answer 1

JavaScript (V8), 67, 57, 56 bytes

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

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

Test cases:

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

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

Answer 2

Bash, 31, 24 bytes

7 bytes saved thanks to @manatwork and @nwellnhof

date -d${1%??}1month +%F

Try it online!

Answer 3

Ruby 49 bytes

d=Date.parse(s);print Date.new(d.year,d.mon,1)>>1

Online repl

Answer 4

PHP, 57 bytes

[$y,$m]=explode('-', $s);$m=++$m%13?:!!++$y;$d="$y-$m-1";

Test cases:

<?php
function next_month($s) {
    [$y,$m]=explode('-', $s);
    $m=++$m%13?:!!++$y;
    return "$y-$m-1";
}

$s = "2018-05-06";
echo next_month($s); // Gives 2018-6-1

$s = "2019-12-20";
echo next_month($s); // Gives 2020-1-1

Answer 5

Red, 39 bytes

t: load s
t/month: t/month + t/day: 1
t

Answer 6

Retina 0.8.2, 33 bytes

T`9d`d`.9*(-12)?-..$
-23|-..$
-01

Try it online! Link includes test cases. Explanation:

T`9d`d`

Increment with wrap-around...

.9*(-12)?-..$

... the last non-9 digit, plus trailing 9s, in either the year or the month, depending on whether the month is 12. (Digits in the following date parts also get incremented.)

-23|-..$
-01

Replace the day with 01. Also, if the month was 12 before, it will be 23 now, and also needs to be reset to 01.