Duration add/subtract doesn't take into account how many days in a month

[alexander-ulaev]

Describe the bug
methods add and subtract with duration object doesn't take into account how many days in a month

// actual

dayjs('2023-02-01').add(dayjs.duration(1, 'month')) === "2023-03-03"
dayjs('2023-03-01').subtract(dayjs.duration(1, 'month')) === "2023-01-30"

Expected behavior

// expected

dayjs('2023-02-01').add(dayjs.duration(1, 'month')) === "2023-03-01"
dayjs('2023-03-01').subtract(dayjs.duration(1, 'month')) === "2023-02-01"

Information

• Day.js v1.11.8
• OS: Ubuntu 23.04
• Browser: chrome 114.0.5735.133
• Time zone: GMT +04:00 Europe/Samara