Last day of month?
https://javascript.info/task/last-day-of-month
JavaScript: Find out the last day of a month - w3resource
https://www.w3resource.com/javascript-exercises/javascript-date-exercise-9.php
用JavaScript获取当月第一天和最后一天 | OurJS
http://ourjs.com/detail/593658adf1239006149616c1
javascript - How to get last day of the month - Stack Overflow
https://stackoverflow.com/questions/1924815/how-to-get-last-day-of-the-month
JavaScript获取月份最后一天_A_Runner的博客-CSDN博客
https://blog.csdn.net/A_Runner/article/details/80332559